Like others have said, you really don't need antivirus software on a Mac. If you want extra piece of mind; however, you can download free programs from the App Store such as Virus Barrier Express or Bitdefender Virus Scanner. Sophos Antivirus is also a decent program which scans for both Windows and OS X viruses, which may or may not be relevant to you if you're working with Windows PCs or Bootcamp since Windows viruses that are harmless to OS X may be transferred to them. I find that those apps aren't obtrusive or hard to deal with like many other antivirus apps or even trial versions of those premium products like Intego or Norton Antivirus, which really are a waste of your money. In all seriousness, you may not even have a use for the free apps either because 99.99% of the time all you're gonna be doing is running a scan to see the 'No viruses found' indicator. It's a shame too, 'cuz I always wanted to click on the 'Kill Virus' button. 😀
Common sense will also go a long way, but if you know you're attracted to risky areas of the web like porn and illegal movie streaming sites then you might enjoy the extra piece of mind.