Especially for Windows users that just switched to Mac these are the most common questions; What's the best antivirus for Mac, where can i get is and do i really need it?
There are no Mac OS X virii out there yet so any form of antivirus is unnecessary and a waste of money. However there are some situations where having antivirus software on your Mac can be useful.

1. You use the Mac in a windows network.
If there is a virus going around in the PC network it will not affect your Mac, however some schools and companies demand that every machine on the network are equip with antivirus software whether it's a PC or a Mac. The antivirus on the Mac will pick up on a PC virus and heal/delete the infected file, in turn you can warn the IT dept or your coworker about their infected email that has been going around. Mac will be the hero ;)

2. You use Windows.
Either through Boot Camp, VM Ware or Parallels you might be using Windows on your Mac. In Windows itself an antivirus application has to be installed but if you want to be sure no files outside of the Windows realm have been infected, antivirus on the Mac could come in handy.

3. You refuse to believe there are no known virii for Mac.
That's fine, install antivirus software to put your mind at ease.
Although it is likely to slow down your Mac one way or another, it can't hurt.

Then what should i use?
Stay away from Norton whatever you do! For all different kinds of reasons that are really not pleasant, take my word for it. The good reputation they might have in Windows world, they definitely do not share in Apple world.
Personally i recommend ClamXav.
A very light antivirus application that only hogs system resources if a scan is active. It doesn't nest itself into your system like Norton does and therefore is easy to uninstall if needed.

For more information on ClamXav, click here.

There are others out there such as McAfee and Intego that will do as advertised, keep your Mac virus free, however they will slow your Mac down much more. The biggest advantage ClamXav has over all the others.... it's free :)