Tripwire

Tripwire is a good example of a file integrity assessment tool. It basically monitors important
files on your box to see if any of them have changed size (a probable sign of an installed
backdoor or rootkit). Its pretty well respected in the security industry due its age (circa
1992) and reliability, and with the recent announcement that all future versions will be
open-source (lesser GPL), its likely to remain popular and current.

http://www.tripwire.com