Mac OS X. ;-)
Honestly, I haven't had an anti-virus program since they stopped development on Disinfectant.
I used the anti-virus that came with .Mac until they quit supporting it. It caught a few pages that had Windows malware, but nothing that would affect the Mac system. I keep the firewall up, don't surf for porn (which, if I understand correctly, is where a BUNCH of the web page malware is located), and have declined to assist any Nigerian princes in moving their money to the US. I also don't run in Administrator mode, and never install ANYTHING that asks for a password, unless it's from a trusted source. It's worked so far.
Literally almost nothing. I believe OS X Server ships with ClamAV, but that's more a precaution than a necessity. Norton is available for OS X, but as there are no OS X viruses, it's unnecessary.
Installing a virus scanner will give you peace of mind, I suppose, but it's still not necessary on a regular OS X install.