None is required. OS X has built-in anti-malware software. OS X also has a built-in firewall you can enable via Security & Privacy preferences. These preferences also control Gatekeeper which can be used to prevent software from being downloaded to the computer unless it comes from a safe site such as the Mac App Store.
Helpful Links Regarding Malware Protection
An excellent link to read is Tom Reed's Mac Malware Guide.
Also, visit The XLab FAQs and read Detecting and avoiding malware and spyware.
See these Apple articles:
If you require anti-virus protection I recommend using ClamXav.