iAntiVirus is a free antivirus solution for Mac, offering most of what a regular user needs. As the official site says, this application offers powerful malware protection (viruses, worms and trojans) - detection and removal-, spyware, adware and dialer protection, real-time protection via "IntelliGuard Real-Time Protection", free support by community and free "Smart Updates".
It offers just a basic interface, and uses very little resources as I even don't feel it working in the background. These are what the free version offers. If you need it for business and commercial purposes, 24/7 dedicated and phone support, paid version is also available.
Consult the official site for more details: http://www.iantivirus.com/.