Network Tools
Ethereal - http://www.ethereal.com
Ethereal is a powerful packet sniffer. It has an incredibly easy to use GTK+ interface, and decoders of every protocol imaginable. It is more useful for Network/Protocol Analysis than for sniffing.
Tcpdump - http://www-nrg.ee.lbl.gov/nrg.html
The original and best. Tcpdump is a power command line utility similar to Ethereal. It is supported on many platforms, including the win32 version, windump. Again, it is most useful for Network Analysis.
ADM Archive
This is an archive of the well known ADM Crew's tools. It includes ADM's Ultimate SMB Exploit, the ADM Crew demo for Linux, ADM's spoofing library, and of course, ADMsniff.
Dsniff - http://www.monkey.org/~dugsong/
Dug Song's dsniff is a powerful sniffer/decoder. It has password decoders for many popular protocols including AIM(TOC), telent, ftp, IMAP, and more. Additionally, many smaller "helper" apps are bundled, including an ARP spoofer, DNS spoofer, and https and ssh 'man in the middle' attackers. This is one of my favorite bundles of tools.
Ettercap - http://ettercap.sourceforge.net
Ettercap is an extremely powerful multi use tool written in ncurses. I personally don't like its confusing, monolithic design. However, it is quite amazing what even a beginner can do with it.
Fragrouter - http://www.anzen.com/research/nidsbench/
Fragrouter is a fun program that sends fragmented IP packets. The fragments can be any size you like, you can even set the data field to _ONE_ octet. If you can't guess why this would be useful, don't bother downloading it!
Sniffit - http://reptile.rug.ac.be/~coder/sniffit/sniffit.html
Sniffit, your classic network sniffer. It is not stateful, and will re-assemble TCP streams for you. It also includes a handy dandy ncurses interface, which is not used by default.
