ClamXav is a free virus checker for Mac OS X. It uses the tried, tested and very popular ClamAV open source antivirus engine as a back end.
Back in the days before OS X, the number of viruses which attacked Macintosh users totalled somewhere between about 60 and 80. Today, the number of viruses actively attacking OS X users is…NONE! However, this doesn’t mean we should get complacent about checking incoming email attachments or web downloads, for two reasons. Firstly, there’s no guarantee that we Mac users will continue to enjoy the status quo, but more importantly, the majority of the computing world use machines running MS Windows, for which an enormous quantity of viruses exist, so we must be vigilant in checking the files we pass on to our friends and colleagues etc. For example, if you’re a wise person and you’ve turned MS Office’s macro support off then you’re not going to notice that virus which is hiding inside this month’s edition of Extreme Ironing.doc which your friend sent you. If you then forward that document to a less wise person who has not turned off the macro support, then you have most likely just sent him a shiny new Pandora’s Box with a sign saying "Open this end"!
Flippancy aside, I’m sure you get the idea: check the file before opening and/or sending it on to someone else. This gives you the opportunity to avoid the file altogether or at least copy and paste any vital information into a new document and send that instead. Don’t forget, if you run VirtualPC you can still become infected and lose valuable data on your Mac even though technically you’re running Windows inside a sandbox. VPC will run any application you tell it to, virus or no virus, it doesn’t know the difference. You can protect yourself slightly by not using VPC’s "shared folders", but that’s a useful feature which you shouldn’t have to be without.