Superscan
Powerful Windows TCP port scanner, pinger, resolver.

OpenVAS
Open Vulnerability Assessment System (OpenVAS) is a network security scanner with associated tools like a graphical user fontend. The core is a server component with a set of network vulnerability tests (NVTs) to detect security problems in remote systems and applications. OpenVAS server is a forked development of Nessus 2.2. OpenVAS-Client is a terminal and GUI client application for OpenVAS and Nessus. It implements the Nessus Transfer Protocol (NTP) and is a successor of NessusClient 1.X.

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Wireshark
Free software protocol analyzer, or "packet sniffer" application, used for network troubleshooting, and analysis. It is the de facto (and often de jure) standard across many industries and educational institutions.

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Netstumbler
A tool for Windows that facilitates detection of Wireless LANs using the 802.11b, 802.11a and 802.11g WLAN standards.

SARA
is a testing and reporting toolbox that collects a variety of information about networked hosts (Vulnerability Scanner). SATAN was designed to help systems administrators automate the process of testing their systems for known vulnerabilities that can be exploited via the network.

NMap
is a security scanner. Just like many simple port scanners, Nmap is capable of discovering passive services on a network despite the fact that such services aren't advertising themselves with a service discovery protocol. In addition Nmap may be able to determine various details about the remote computers. These include operating system, device type, uptime, software product used to run a service, exact version number of that product, presence of some firewall techniques and, on a local area network, even vendor of the remote network card.

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Open Source Tripwire
Security and data integrity tool useful for monitoring and alerting on specific file change(s) on a range of systems.Cryptographic hashes are employed to detect changes in a file without storing the entire contents of the file in the database.

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OpenNMS
Network monitoring platform for enterprise-grade networks featuring service polling, data collection, thresholding, event management, alarms and automations and notifications. The goal is for OpenNMS to be a truly distributed, scalable platform for all aspects of the FCAPS network management model, and to make this platform available to both open source and commercial applications.

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Firewall
Dedicated Linux distributions providing out of the box firewalls in order to protect IP-networks in an easy way.

• redWall Firewall - bootable CD-ROM Firewall based on Linux. It's goal is to provide a feature rich firewall solution. Configuration is currenty stored on a floppy/USB Memory Stick/Harddrive.
  
• IPCop - Linux-distribution, with dedicated functions like Router, Firewall, Proxyserver, DHCP-Server, Caching-Nameserver, Traffic-Shaping, VPN und Dynamic DNS and Snort.
  

Personal Firewall
A personel firewall is software to control applications and network traffic on a computer enforcing rule based policies. Diffrenet to common firewalls a personal firewalls has to support the access control of code execution and the conjunction of application and network activities on a computer. A classical firewall is foccused on controling network traiffic for a network.

• Comodo Firewall Pro -Personal firewall providing protection on a Microsoft Windows operating system to control application, execution and network traffic.
  
• TuxGuardian - Personal firewall for Linux enabeling to enforce access control policies to the network resources in order to identify and control every application that tries to access the network.
  

Firefox
Award-winning, free Web browser to browse the Web with confidence with protection against viruses, spyware and pop-ups. It has an intuitive interface supporting tabbed browsing, live bookmarks, integrated search, automated updates and many plugins to custumize a individual functionality. It's mayor advatange are the security features like pop-up blocking, protection against worms, viruses, spyware, clearing private data and so on.
In order to enhance security using the WWW the following plugins are very helpful:
Adblock, NoScript, Secure Login, PasswordMaker

Opera Mini
Opera Mini is a java driven web browser designed primarily for mobile phones, but also for smartphones and personal digital assistants capable of running Java ME applications. Opera Mini requests web pages through a proxy server operated by the Opera Software company's , which process and compress them before relaying the pages back to the mobile phone. This compression process makes transfer time about two to three times faster, and the pre-processing smooths compatibility with web pages not designed for mobile phones. Moreover the pre-processing reduces the cpu uitlisation memory consumtion on the mobile device.

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HTTrack
Offline browser utility which mirrors Sites to local storage. It allows to download a World Wide Web site from the Internet to a local directory, building recursively all directories, getting HTML, images, and other files from the server to your computer. HTTrack arranges the original site's relative link-structure.

Thunderbird
Email client delivering safe, fast, and easy email, with intelligent spam filters, quick message search, and customizable views. Simple to use, powerful, and customizable, Thunderbird is a full-featured email application. IMAP and POP mail protocols, as well as HTML mail formatting are supported. Built-in RSS capabilities, powerful quick search, spell check as you type, global inbox, deleting attachments and advanced message filtering round out Thunderbird's modern feature set.
In order to enhance security handling emails the following plugins are very helpful:
Allow HTML temp, Sender Verification Extension

ChatZilla
IRC-Client enabling people to talk to each other exchaning text messages via the Internet in real-time sessions using virtual rooms. To do so the client connects to an IRC-Server. Some more information about Internet Relay Chat (IRC) is provided on the web site LIVING INTERNET.

VPN-Client
IPsec Remote Access VPN Client for Windows and UNIX oprating systems. It was originally developed to provide secure communications between mobile Windows hosts and open source VPN gateways that utilize standards compliant software such as ipsec-tools, OpenSWAN, FreeSWAN, StrongSWAN, isakmpd. It now offers many of the advanced features only found in expensive commercial software solutions and provides compatibility for VPN appliances produced by vendors such as Cisco, Juniper, Checkpoint, Fortinet, Netgear, Linksys, Zywall and many others.

• Putty - Telnet and SSH for Win32 and Unix platforms with an xterm terminal emulator.
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• PocketPuTTY - PuTTY port for Windows CE
  

WinSCP
FTP, SFTP and SCP Client with graphical user interface. WinSCP is based on PuTTY.

TFTP32
Tftpd32 includes DHCP, TFTP, SNTP and Syslog servers as well as a TFTP client for Win32 plattforms. It is the tool of choice to configure dedicated network devices like router, switches and so on using a windows operating system.

PumpKIN
TFTP server and TFTP client, which implements TFTP according to RFC1350. It also implements block size option, which allows transfer of files over 32MB, as well as transfer size and transfer timeout options described in RFC2348 and RFC2349.
The primary use of PumpKIN is maintenance of the network equipment (such as router firmware upgrade) that fetches images from TFTP server.

Peer-To-Peer File Sharing
A pure peer-to-peer network does not have the notion of clients or servers, but only equal peer nodes that simultaneously function as both "clients" and "servers" to the other nodes on the network. This model of network arrangement differs from the client-server model where communication is usually to and from a central server. The first generation of peer-to-peer file sharing networks had a centralized file list provided by a central server. The second generation create a network without a central index server using distributed hash tables (DHTs) by electing various nodes to index certain hashes. The third generation of peer-to-peer networks are those that have anonymity features built in.

• MLDonkey - different to other P2P-Clients MLDonkey hos no build in GUI. User interaction is enabled by HTTP, FTP or a specific MLDonkey-GUI-Protokoll, which is implemented by several GUI-Clients. Some more information is provided at FAQ
  Supported networks:
  eDonkey2000, Overnet, Gnutella, Gnutella2, FastTrack (KaZaA), BitTorrent, Kademlia, Soulseek, Direct Connect.
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• Sancho - GUI-Client for MLDonkey.
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Pidgin
Multi-protocol Instant Messaging client that can work with AIM, Bonjour, Gadu-Gadu, Google Talk, Groupwise, ICQ, IRC, MSN, MySpaceIM, QQ, SILC, SIMPLE, Sametime, XMPP, Yahoo!, Zephyr.

Skype
Skype is a little piece of software that lets you make free calls to anyone else on Skype, anywhere in the world. Major advantages are excellent audio quality and the support of diffrent operating system plattforms (Windows, Linux, MacOS, PocketPC) and easy to use user interface. Next to voice calls skype supports video streams, chat and file transfers as well as confernce calls. It is even possible to call regular phones or being called by phones.

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SIP-Softphones
A SIP-Softphone is a softawre which enables phone calls from computer to computer or from a computer to a phone connected to a public switched telephone network. A comparison of diffrent softphones is available at wikipedia: Comparison of VoIP software.

• Express Talk - SIP-Softphone with a phone and log book and ability to record phone calls.
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• Linphone - simple and reliable SIP-Softphone with voice, video, and text instant messaging.
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• LYNXPHONE - SIP-Softphone with encryption support.
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• freeSSH - native SSH/SFTP server for Windows
  
• Sysax Multi Server Free - SFTP and SSH Server which also provides FTP, FTPS und HTTPS file transfer support. It only can be used by one user at a time
  
• CopSSH - complete reference package of OpenSSH for windows using Cygwin
  

• RealVNC - RealVNC is a direct descendant of the orginal develped VNC at the Olivetti & Oracle Research Lab. The free version does transmit data unencrypted. Using the software to control computers via the internet the vnc data should conveyed through a secure tunnel like a VPN or ssh-tunnel.
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• other VNC derivates
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• .NET VNC Viewer - VNC Viewer for Windows CE
  

Syslog Daemon
Syslog Server for Windows.

TFTP Server
TFTP Server for Windows.

ddclient
Dynamic Dns Client written in Perl. It is very flexible in terms of DDNS-Service provider support and and IP detection methods.

OpenVPN
OpenVPN is a full-featured SSL VPN solution. It implements OSI layer 2 or 3 secure network extension using the industry standard SSL/TLS protocol, supports flexible client authentication methods based on certificates, smart cards, and/or 2-factor authentication, and allows user or group-specific access control policies using firewall rules applied to the VPN virtual interface.

• OpenVPN GUI for Windows - Windows Control-Window which can be opened from an icon located in the notification area (the area on the right side of the taskbar) from which you can control OpenVPN to start/stop your VPN tunnels, view the log, change your password and other useful things.
  
• OpenVPN-admin - Webmin Plugin for openVPN.
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OpenCA
At OpenCA tools are provided to setup and operate a "Certification Authority" (CA ). A CA issues and manages digital certifcates and represats the base of a "Public Key Infrastructure" (PKI). PKIs bind digital identity to physical entities providing the base for symetric and asymetric crptography application in modern IT Infrastructures.