Spot The Difference is a file integrity checker. Its goal is to detect signs of
intrusion by looking for suspicious changes in system files. Crackers, in fact,
to do their evil or just to make sure they can work their way back into the
system, often change some configuration files, executables and/or log files
(usually with rootkits); thus leaving signs of the break-in.

WWW: http://www.kernel-panic.it/software/stdiff/