No there is no need to run antivirus software. Rather than reading people's opinions though (such as mine) let's see what Apple has to say.
*Doesn't get PC viruses.*
*Designed with security in mind, Mac OS X isn’t plagued by constant attacks from PC viruses and malware. Likewise, it won’t slow you down with constant security alerts and sweeps. Every Mac is secure right out of the box, so you can safely go about your work — or play — without interruption*
So that is the definitive word on PC virus's so how about Mac virus's, well the good news there are zero, yes zero Mac virus's. Therefore if Mac's aren't plagued by PC virus's and there are zero Mac virus's that makes using antivirus software unnecessary. That being said if you are still a disbeliever then download and use
ClamAV, it's free and if you research these forums will find it's generally accepted as the tool to use.