Nessus : Premier UNIX vulnerability assessment tool
Nessus was a popular free and open source vulnerability scanner until they closed the source code in 2005 and removed the free "registered feed" version in 2008. A limited “Home Feed” is still available, though it is only licensed for home network use. Some people avoid paying by violating the “Home Feed” license, or by avoiding feeds entirely and using just the plugins included with each release. But for most users, the cost has increased from free to $1200/year. Despite this, Nessus is still the best UNIX vulnerability scanner available and among the best to run on Windows. Nessus is constantly updated, with more than 20,000 plugins. Key features include remote and local (authenticated) security checks, a client/server architecture with a GTK graphical interface, and an embedded scripting language for writing your own plugins or understanding the existing ones.
Wireshark : Sniffing the glue that holds the Internet together
Wireshark (known as Ethereal until a trademark dispute in Summer 2006) is a fantastic open source network protocol analyzer for Unix and Windows. It allows you to examine data from a live network or from a capture file on disk. You can interactively browse the capture data, delving down into just the level of packet detail you need. Wireshark has several powerful features, including a rich display filter language and the ability to view the reconstructed stream of a TCP session. It also supports hundreds of protocols and media types. A tcpdump-like console version named tethereal is included. One word of caution is that Ethereal has suffered from dozens of remotely exploitable security holes, so stay up-to-date and be wary of running it on untrusted or hostile networks (such as security conferences).
Snort : Everyone's favorite open source IDS
This lightweight network intrusion detection and prevention system excels at traffic analysis and packet logging on IP networks. Through protocol analysis, content searching, and various pre-processors, Snort detects thousands of worms, vulnerability exploit attempts, port scans, and other suspicious behavior. Snort uses a flexible rule-based language to describe traffic that it should collect or pass, and a modular detection engine. Also check out the free Basic Analysis and Security Engine (BASE), a web interface for analyzing Snort alerts. Open source Snort works fine for many individuals, small businesses, and departments. Parent company SourceFire offers a complimentary product line with more enterprise-level features and real-time rule updates. They offer a free (with registration) 5-day-delayed rules feed, and you can also find many great free rules at Bleeding Edge Snort.
Netcat : The network Swiss army knife
This simple utility reads and writes data across TCP or UDP network connections. It is designed to be a reliable back-end tool that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and exploration tool, since it can create almost any kind of connection you would need, including port binding to accept incoming connections. The original Netcat was released by Hobbit in 1995, but it hasn't been maintained despite its immense popularity. It can sometimes even be hard to find nc110.tgz. The flexibility and usefulness of this tool have prompted people to write numerous other Netcat implementations - often with modern features not found in the original. One of the most interesting is Socat, which extends Netcat to support many other socket types, SSL encryption, SOCKS proxies, and more. It even made this list on its own merits. There is also Chris Gibson's Ncat, which offers even more features while remaining portable and compact. Other takes on Netcat include OpenBSD's nc, Cryptcat, Netcat6, PNetcat, SBD, and so-called GNU Netcat.
Source code Metasploit Framework : Hack the Planet
Metasploit took the security world by storm when it was released in 2004. No other new tool even broke into the top 15 of this list, yet Metasploit comes in at #5, ahead of many well-loved tools that have been developed for more than a decade. It is an advanced open-source platform for developing, testing, and using exploit code. The extensible model through which payloads, encoders, no-op generators, and exploits can be integrated has made it possible to use the Metasploit Framework as an outlet for cutting-edge exploitation research. It ships with hundreds of exploits, as you can see in their online exploit building demo. This makes writing your own exploits easier, and it certainly beats scouring the darkest corners of the Internet for illicit shellcode of dubious quality. Similar professional exploitation tools, such as Core Impact and Canvas already existed for wealthy users on all sides of the ethical spectrum. Metasploit simply brought this capability to the masses.