First of all, there are no viruses on OS X currently. Secondly, there are only a few trojans and malware that can be easily avoided if you have common sense. If you have Lion and even snow leopard, it has built in malware prevention, so really don't need third party applications. If you do want to use one though, ClamXav is good since it also detects a windows malware and it's useful if you spend time sending files that may be affected to Windows computers.
