In my quest to get rid of closed source products, I've been moving over quite a few things. One important one is virus checking as you don't want an exploit where they take your virus scanner out. There is only one for the Mac that appears to be open source (just like there appears to only be Thunderbird for Mail), so I use ClamXav.
But, you have to download it from their site because:
The App Store version doesnt contain ClamXav Sentry, so it is not able to watch your computer for new files and scan them as they arrive.