Ad-aware In our opinion, it's the best software available to wipe all the spyware on your machine. With it's adware auto alert software, which runs in your system tray, keeps you informed of any changes being made to your registry or any suspicious process running.
Spybot Search & Destroy Excellent free software to remove spyware from your computer and it comes with many useful features. RECOMMENDED!
Scumware.com Very good page for general information about Spyware, with some useful information on how to manually remove some of the worst spyware.