Most malware targets Windows machines and vulnerabilities (because there's more of them), but that doesn't mean Macs are immune from security issues, especially of the "click on this tempting site" variety. Sophos is offering a free, unrestricted version of its Mac anti-virus package for home users concerned about security issues.
Built off the multi-platform AV package which it sells to enterprise customers, the Mac release doesn't require registration and doesn't have an expiry date. A free software package from a reputable supplier beats a smug sense of "this could never happen to me" any day. We don't have a Mac running at Lifehacker AU HQ, so if you install this, tell us about the experience in the comments.