I myself use the free Avast and Winpatrol programs, but there's a ton of free stuff. Threatfire, Avira, Spyware Terminator, Comodo (full suite), Spybot, Spyware Blaster, Spyware Guard, Bit Defender, A-Squared (they have a ton of different apps, like anti-dialer, anti-rootkit, anti-malware), ClamWin (scanner) and hell, there's even a Windows Defender in case you want on directly integrated into windows (It comes activated by default on Vista, but it's optional on Xp. I think.).
I've been tempted to move to the Comodo Suite due to it being all-in-one, but I've been with Avast for years so it's hard for me to let go.