Free ones include Windows Defender Beta 2, Lavasoft Ad-aware and
Spybot Search and Destroy.
There are others which offer free scanning, but no removal. These
are Webroot Spysweeper and Spyware Doctor from PC Tools.
There are some which offer full features for a limited time. One of
them is a-squared anti-malware. You can use it for 30 days.
There are some which offers full features for 30 days, after that,
it will offer basic features. Ewido is one of them.