Spybot Search and Destroy is free for home use, as are all the programs mentioned in this thread: Ad-Aware, AVG Anti-Spyware, a-Squared, SpywareTerminator and SuperAntiSpyware.
SpywareDoctor from PC Tools is not free. It has a free trial but won’t remove malware found.
http://www.pctools.com/
Make sure you are not looking at a scam page trying to sell knock-off programs with similar names that won’t work or may even charge money for doing nothing.
The links in this thread are good: be careful when Googling because the scams can come up. Searching for ‘Spybot’ especially produces lots of scam links.
Here’e the link for Ad-Aware:
http://www.download.com/3000-2144-10045910.html