Neal,
Spybot S&D gets my vote! It located half a dozen unwelcome guests in my system, and even gave a brief description of the companies involved. Very comprehensive. I'm running it every couple of days and nearly always find that new intruder or two.
It's almost worth getting Spybot on your system for the chance to read the Licence Agreement alone. Clear, straightforward language. With a couple of jokes as well. Donations on the way to spybout and D_A_L. Thanks!