Adaware do still provide a free version, no need to pay for it unless you want the resident shield.
I use SpywareTerminator, which is freeware, and has a resident shield and HIPS, and is, I believe, vastly superior. Apart from Windows Defender, it's the only free antispyware that includes realtime protection.
Other worthy scanners with free (demand) versions available include Superantispyware (excellent) AVG AS, Asquared, and Spybot, of course.
AVG also make a free rootkit remover, as do a couple of others.
SpywareBlaster is an excellent and popular tool for blocking known bad sites and activex, uses no resource once the block is applied. Winpatrol guards several system settings not quite in realtime, and every time something changes one of those settings, homepage for example, alerts the user.
http://www.spywareterminator.com/
http://www.superantispyware.com/
http://www.emsisoft.com/en/software/download/
http://free.grisoft.com/doc/20/lng/us/tpl/v5
http://www.javacoolsoftware.com/
http://www.winpatrol.com/