avast! Free Antivirus for Mac protects you from Mac-targeting viruses and other malware attacks.

Certified antivirus and anti-spyware engine
Our scanning engine's unique technologies protect against viruses, spyware, and other malicious agents.

WebRep plugin to help protect against phishing scams
WebRep browser plugins provide greater protection against phishing sites, including heuristic detection of new phishing sites.

Three types of real-time malware scans
Run real time malware scans on your file system, email tools, and web-browsing activities (web pages, downloads, and java scripts).

System requirements
Operating Systems Supported
• Mac OS X 10.5.x (Leopard)
• Mac OS X 10.6.x (Snow Leopard)
• Mac OS X 10.7.x (Lion)
• Mac OS X 10.8.x (Mountain Lion)
Minimum Hardware Requirements
• Intel-based Mac

Please note that avast! will not run properly if you have other antivirus software installed.

http://www.avast.com