Although the mac is reputed to have few or no viruses (a few have been made as proof of concept but they aren't really comparable to what windows users experience), it's better to take precautions. I've recently installed ClamXav, which came on one of the MacFormat DVD's. It's a freeware Anti-Virus for Mac. It comes from another project called ClamAv, which is for other platforms. It works well and you can also set up schedules to do a Virus scan, as well as a Sentry, which can watch certain directories always (for example your mail attachment or Downloads folder).
Now, although the risk is less for us mac users, we should be responsible and allow this to apprehend Viruses that could otherwise be passed on in an email to our colleagues or friends. It's like paying your TV license, it's the right thing to do.