I haven't had any issues with Malware on my Mac, so I'm not sure how well these hold up in a crisis, but...
I use something called iAntiVirus, and have been very happy with it. It doesn't use a ton of resources, and scans are relatively quick.
I've heard good things about something called Sophos Anti-Virus for Mac, which is available at the Apple Store online.
Both are free:
Cnet page for iAntivirus
Sophos on the Apple website