I've been using Sophos' freebies on my Macs, but the latest OS X updates seemed to have some conflict that kept hanging everything. I uninstalled, and tried ClamXAv, and it worked fine, but was a bit clunky, and lacking . I'm now trying Avast's freebie, and it seems to be lightweight and trouble-free (so far). Plenty of free options, so no excuse not to be protected.
"Not everything that can be counted counts, and not everything that counts can be counted." Albert Einstein