If you're into computer forensics investigations you can check out the
SANS Investigative Forensic Toolkit (SIFT) Workstation, which is basically to forensics what Backtrack is to penetration testing. SIFT is a virtual machine that comes preloaded with all kinds of open source forensic software.
Then there is also
REMnux, which is another virtual machine, but contains all kinds of open source software for reverse engineering malware. Lenny Zeltser created it, and I believe it's used to teach his SANS GREM class. There is quality information on his
blog as well if you're interested in malware analysis.