The limited number of viruses that target Mac OS X often leads Mac users into a false sense of security. Although there are few Mac viruses out there now, that doesn't mean there will never be a virulent form of malware that threatens the Mac. And the lackadaisical approach of assuming that Macs can't be infected by a virus increases the chances of widespread infection and damage when (not if) such a virus is developed and released into the wild of the Internet.
Besides, even if it doesn't affect you directly, you might receive and inadvertently pass on a Windows-specific virus. The most common of these are macro viruses found inside Office documents -- some of which have a limited ability to affect machines with a version of Office for Mac that has macro support enabled.
Finally, if you are running Windows -- either in a dual-boot configuration with Apple's Boot Camp or under a virtualization tool such as Parallels Desktop -- your Mac is just as susceptible to viruses as any PC. In fact, if you have a Windows operating system on your Mac, you should really consider virus protection for both Mac OS X and Windows.
When it comes to antivirus programs for the Mac, a good open-source option is the Unix-based ClamAV (which works with Mac OS X but is command-line based) and its Mac graphical user interface port ClamXav.