I am a big fan of
Spywareblaster.
It doesn't scan for currently running spyware, but keeps it from ever installing. Sort of like an antivirus patch for spyware. And like all the best antispyware programs, it's free and updated often.
For active scanning, I happen to use Adaware. Although I don't often turn up anything more malicious then tracking cookies these days.
-Andy A.