People here like to say that an antivirus program isn't necessary on Macs; if you want to have one just for peace of mind, try ClamXav. It's a free, lightweight app that won't interfere with your computer operations.
Yup, I use ClamXav, and am a relatively new Mac user. I know I probably won't and will scarcely run into a virus for the Mac, but considered I was raised in a PC, virus-freak family (and I've done quite well protecting my PC) its hard not to at least have an anti-virus software on my Mac. But I'm also considering uninstalling it.
EDIT: not because ClamXav is bad that I may uninstall it but because it won't be needed.