You really don't need it (except on your Parallels and Bootcamp installs). I switched in Jan 2006, and all my antivirus ever found was Windows viruses that Norton never seemed to catch in my email
Anti-Virus -- ClamXAV -- Clam is open source, free, and excellent in quality. Good enough that the next version of OS X Server (Leopard Server) will use it as its core server-level AV technology. There aren't really OS X viruses in the wild, but this is very effective at identifying and eliminating Windows viruses, and it is regularly updated. I have it on both of my Macs but honestly do not enable the background scanner nor do I do regular scans. When I last did a scan, the only viruses I found were in junk mail in my Mail trash folder that I had not yet deleted (relatively old junk mail from before server-side scanning became efficient). So there wasn't even any real risk of me transmitting a Windows virus to a Windows user.
Spyware/Adware -- this is harder, because it doesn't really properly exist on Macs at this point, and even if Windows ones found their way onto your computer, there's no way for them to transmit to other computers... MacScan has a free trial version. I installed it on one of my Macs and ran it once and found it to be a waste of time.