Yes, there is Mac OS X malware in the wild (no viruses, just a handful of trojans), but it can all be easily avoided by practicing safe computing. There is no need to install any antivirus app, as none of those apps will provide any greater protection than practicing safe computing. I especially recommend avoiding Sophos, as it can actually increase a Mac's vulnerability, as described here and here. 3rd party antivirus apps are not needed to keep a Mac malware-free, as long as the user practices safe computing, as described in the following link.2) Some will argue this, but there is malware on the Mac, so I would recommend installing an antivirus program. I personally like Avast and Sophos and they are both free.
If anyone insists on running antivirus for some reason, ClamXav (which is free) is one of the best choices, since it isn't a resource hog, detects both Mac and Windows malware and doesn't run with elevated privileges.