Short answer:
Virus free. You don't need any virus protection software. Just common sense.
Long answer:
There are a few trojans, but they're rare. You need to download them yourself, launch their installer, and supply your password to install them. Don't install software you don't trust, or that you downloaded from a shady site (i.e. porn sites asking to install a new video codec.)
There aren't many good OS X antivirus products anyway. Most only pick up Windows threats, or don't work, or harm your system and crash it, or are overpriced.
If you want to stop yourself from passing on Windows only threats, you could try ClamXAV, which is free.
iAntiVirus is also free, and has a database of only OS X threats.
Unfortunately, iAV may only detect the trojan installer packages, so if you have already accidentally installed them, it won't pick it up. This is because the threat definitions only specify the installer packages, not the actual installed components. I emailed the developer, they have now fixed the definitions for the keylogger I specified, but I'm not sure about the other threats they ahve in their database.