This topic has been brought up countless times on the forums. Search for the word "virus" and you'll get quite a few hits.
Here's one:
http://discussions.apple.com/thread.jspa?messageID=9244196�
Limnos' post is probably the one to look at.
Many of the long-time users here agree that the most effective anti-spyware/malware/adware/what have you protection is for the user to pay attention to what he/she is doing.
-Create a standard user account and use that for daily tasks. Use the admin account to perform updates.
-Don't download files from sites you don't know. Don't install anything unless you know what it is.
We did use the Virex anti-virus system years ago for one of our Macs. It was running OS 7 (ancient history, really). None of our current Macs, all of which are running OS X Tiger or Leopard, have any anti-virus software installed, and we haven't had any problems.
If you do choose to install anti-virus, the only real reason to do so is to prevent the spread of any Windows viruses that make their way onto your machine.
~Lyssa