Spybot is free click here are you perhaps thinking of the request for a donation? Its asking you for a donation, you don't have to give on if you don't wish to.
Other free ones include Superantispyware, AdAware 2007, AVG antispyware, theres loads of others.
I'm using Superantispyware and Adaware 2007 just now. Only thing to watch is that you do download the free ones as theres paid ones as well.