In General 3rd Party AV Software is Not Required as Mac OS X tends to look after itself.
See Here > Antivirus Discussion
The Safe Mac > http://www.thesafemac.com/mmg/
In addition to Allan's reply, OS X has got its own security systems, so you don't need any third-party security app. Also, if you install an antivirus, it shouldn't be a commercial antivirus. See > http://www.thesafemac.com/mmg
Apart from that, we only recommend ClamXav and Sophos as antivirus for Macs, and I recommend you to install one of them if you are going to use your Mac to copy files to an external drive that you will use with PCs later. That's caused because your Mac isn't affected by Windows viruses, but you can infect other PCs.
One of the best things you can do is to keep your computer updated. The most recent version is OS X 10.8.4, so open > Software Update, and install the most recent version
sophos antivirus for mac is free for home use
also see why I use it
Since there are no virus that can attack a Mac
I truly wish people would stop saying that. That is a small truth that masks the far greater reality. Malware for the Mac is real, and although anti-virus software is still not necessary at this time, people need to know how to protect themselves. Platitudes without any attempt to educate will not serve.