There are a lot of them around! These days a good, up to date virus checker is essential -- a few years ago (before we all used email all the time) it wasn't really a big deal, but if you haven't had a virus by email yet it's only a matter of time . . .
I currently use EZ Antivirus which you can download from http://www1.my-etrust.com/products/info/Antivirus/2
and I'm happy with it. It's easy to install, use and update. Although it doesn't have an automatic updating routine, it's very easy to do every few days. You can also elect to receive an email every time there is a new update to download.
Previously I used Norton Antivirus until I discovered that it wasn't actually updating itself, despite downloading the updates (it works well for other people though), and I've also used MacAfee antivirus until I lost patience with its convoluted install procedure and slow, advertising laden updates.
As far as I can tell, it doesn't make that much difference which product you use though, so long as you do
use it and keep it up to date!