http://osiris.shmoo.com/ - Awesome free data integrity checker (like a free Tripwire) for Linux

http://www.heatscanner.com/ - No one said it had to be free, but the best Vuln scanner out there (OK, I may be a bit biased towards this one) We use this at least once a day.

http://www.gfi.com/lannetscan/ - Not sure if this was mentioned, but a GREAT free tool if your not using Nessus already.