While you may or may not need a virus scanner on a Mac, there's no harm in having one that you can run as and when, perhaps if you're sending a file to a Windows using colleague for instance. Just because a potentially infected file goes totally unnoticed on your Mac, doesn't mean it's not going to wreak havoc on your poor unsuspecting friend's Windows box.
ClamXav is the Mac GUI variant of the open source ClamAV project, and has recently entered the Mac App Store, making it even easier to get on-demand protection. Thanks to the ClamAV antivirus engine, ClamXav will detect both Mac and Windows threats and remove them quickly and easily. You get a Services menu option, which allows you to scan files as and when, be it that PDF you just downloaded or your entire disc. You can define file exclusions and even save your favorite scan locations for frequent scanning tasks. Virus definition updates are of course free, and you can get ClamXav to update when you first start the program or on a set schedule.
While ClamXav may not be the fastest scanner on the block, it's free, gets the job done, and most importantly, won't bog your system down. If you ever share files with Windows users, or just want to make sure you're not going to fall foul of a Mac malware uprising if it ever comes into being, then ClamXav is a great occasional use scanner.