Malware is very rare for OS X.
What makes you suspect it?

That being said, I use
ClamXAV had always been my choice: ClamXav

For general Apple security, nothing beats this resource: Macintosh Security Site - Security for Mac Platform MacOS X Security Firewalls Desktop Network Security secure mac os x Virus Encrpytion PGP macosx

Keep in mind that OSX is inherently very powerful, but unfortunately not very secure by default. Most users are only safe because of the lack of spyware/viruses that are targetted at Macs.

It really doesn't take much to make your Mac into a fortress though.