We all know Macs are less susceptible to malware and viruses, but that does not mean it isn't good to check your system from time to time. I go back-and-forth between my Mac and my Windows machine at school all the time. Even if my Mac is safe, I do not want to potentially carry something over to my Windows machine and vice-versa. Also, given that I have recently discovered the Vuze BitTorrent client, I like to know that the files I have grabbed are safe.
ClamAV is a UNIX-based open-source anti-virus client, particularly designed (but not limited to) email scanning. I installed it via fink and run it via command line. It is really easy to scan individual files, directories, or your whole machine this way. I recently did a scan of my whole home directory, and true to its speciality, it detected two potential problems, both email phishing attempts (for some reason, our school servers get tons of email from people asking for help with 10 million dollar deposits):