At this time there have been no confirmed Mac OS X viruses (subject to the semantic debate about whether the iChat exploit is properly classified as a virus or a trojan horse and not counting the Word macro virus that can affect Office v.X or non-updated copies of Office 2004), very few trojans and no adware or spyware. I therefore do not feel that antivirus or antispyware software is necessary at this time. It's never wise to become complacent, though, so security precautions are not wasted. Such security precautions mostly are common sense: don't download and run files from sites you don't know, don't double-click on attachments in emails you aren't expecting and/or from people you don't know, and make sure you password your system and keep that password secure as well as not having your system available and unattended (which also is a precaution against theft). In addition, it's possible for someone to inadvertently forward a Windows virus or worm to a PC user though you'd have to do it manually and said Windows malware cannot infect the Mac.
In addition, it's possible for someone to inadvertently forward a Windows virus to a PC user though you'd have to do it manually, and said Windows virus cannot infect the Mac.
If you do decide you wish to run antivirus software, I think that the donationware
clamXav should be more than enough provided you remember to scan your system from time to time.
If you are running Windows on your system, either via BootCamp, Parallels Workstation or another solution, then that copy of Windows is subject to all the myriad exploits common to the Windows world, so you need to take full precautions, including running both antivirus and antispyware software.
