Wireshark (#1, 1)
Wireshark (known as Ethereal until a trademark dispute in Summer 2006) is a fantastic open source multi-platform network protocol analyzer. 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 tshark is included. One word of caution is that Wireshark 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). Read 33 reviews.
Latest release: version 1.12.7 on Aug. 12, 2015 (5 months, 2 weeks ago).
Aircrack (#4, 17)
Aircrack is a suite of tools for 802.11a/b/g WEP and WPA cracking. It implements the best known cracking algorithms to recover wireless keys once enough encrypted packets have been gathered. . The suite comprises over a dozen discrete tools, including airodump (an 802.11 packet capture program), aireplay (an 802.11 packet injection program), aircrack (static WEP and WPA-PSK cracking), and airdecap (decrypts WEP/WPA capture files). Read 14 reviews.
Latest release: version 1.1 on April 24, 2010 (5 years, 9 months ago).
Cain and Abel (#6, 3)
UNIX users often smugly assert that the best free security tools support their platform first, and Windows ports are often an afterthought. They are usually right, but Cain & Abel is a glaring exception. This Windows-only password recovery tool handles an enormous variety of tasks. It can recover passwords by sniffing the network, cracking encrypted passwords using dictionary, brute-force and cryptanalysis attacks, recording VoIP conversations, decoding scrambled passwords, revealing password boxes, uncovering cached passwords and analyzing routing protocols. It is also well documented. Read 13 reviews.
Latest release: version 4.9.56 on April 7, 2014 (1 year, 9 months ago).