Windows only: Freeware application RUBotted monitors your computer for the presence of malware zombie bots and alerts you of any suspicious activity. A common and popular form of malware, bots turn an infected computer into a zombie of sorts that will perform tasks in the background when directed by the maker of the virus. Often a network of bots will be used to send spam or participate in denial-of-service attacks. RUBotted monitors your computer for suspicious, bot-like activity and alerts you if it thinks you may be infected.
Granted, a good antivirus app should hopefully detect any malware before it gets to this point, but RUBotted provides a nice second line of defence. It's made to run all the time (and with a 4MB footprint, it's not that bad), but more people will probably run this on a per-case basis to confirm suspicions. RUBotted is a freeware beta application, Windows only.