First and foremost, EVERY system needs an antivirus scanner, regardless of whether it's running Windows, Linux, Mac, or some other esoteric operating system. While Mac and Linux platforms may be immune to infection by 99% of the viruses out there, there is still the possibility of spreading the virus to other users. For example, several Trojan Horses are spreading via P2P (peer-to-peer) networks and are disguised as popular music or video files. A Mac user may download one and never realize it's there. Meanwhile their favorite P2P program (Kazaa, LimeWire, etc.) is sharing it out to users who are vulnerable.
I personally recommend Norton AntiVirus over McAfee, which comes with several PCs. Trend Micro also makes a home-use program called PC-cillin. Be prepared to spend about $30-35 for software and $4/year on virus subscriptions in all cases. However, as the title of this article suggests, there are free alternatives out there!
The first is Trend Micro's HouseCall, an online scanner. I often use this one for troubleshooting and emergency scanning, and I don't recommend it for permanent use as realtime scanning of your system will be impossible. If you feel you have a problem your current AV system (or lack of AV system) is not detecting, surf to housecall.antivirus.com and start a scan. A solid, free program for full-time home use is AVG, made by Grisoft. I've switched to AVG on my Windows 98 machine at home and have been very happy with it. It performs scheduled scans and you can update it as often as you like for free. You will have to register a legitimate email address to receive an unlock code, but I have had no problems with spam, etc., from Grisoft. AVG performs realtime and boot scanning as well as inbound and outbound email scanning. There is a pay version which is relatively inexpensive if you would like the extra doodads and some finer control, but most of them are unnecessary. Running Linux or Mac OS X? Check out RAV AntiVirus. They offer several free downloads for a number of platforms. While I have not tested their software directly, their reputation is solid.