rootkits.
ever wonder if your system is hacked? i've been told about a bunch of applications, but nothing has impressed me like rkhunter has. this application has it all, and it also has very nifty CLI friendly output.
i'd also recommend chkrootkit, but i must say that i'm quite impressed with rkhunter and its ease of installation and use.
i have already set a cron job for this to be run on a regular basis to verify that my system is running smoothly.