Several things for free you can get.
I use the Free spywareguide.com blocklist that sets the killbit in the registry
for known bad CLSIDS: http://www.spywareguide.com/blockfile.php
Doesnt get them all but it's free and stops some of the scum.
They also have a freeware scanner that targets some of the more common ones but not all of them. Good for a quick scan.
I also use the paid version of Ad-aware, Spybot Search and Destroy and the paid version of X-Cleaner. They also have a freebie at http://www.xblock.com/download-freeware.shtml
but it doesn't scan the full database.
The value for me in X-Cleaner is their help desk will work with you one-on-one to make sure your system stays clean. Top notch support although only be e-mail.
I have also been beta testing their new program called X-RayPC. It scans for all hidden and known processes and compares it to spywareguide's database in real time, flagging spyware and adware entries in red and safe controls in green. Very cool stuff, but still in beta. They tell me they plan to release it next month as freeware. You can contact email@example.com
if you want to beta test I think.
Also for CoolWebSearch I use Merijin's http://www.spywareinfo.com/~merijn/cwschronicles.html
it is free and the only one that completely gets rid of CoolWebSearch which is probably the worst piece of spyware trash I have ever seen. Getting rid of it requires a digital enema.