The ones I keep on my USB thumb drive and frequently use...
Latest versions of...
Spybot Search and Destroy
And Esets Sysinspector...which is like a combination of Hijack this and Autoruns..with a few deeper options.
Some updates to my list...
MalwareBytes is still probably my favorite malware cleaning tool. For those computers which get infected by malware which prevents updating your existing tools, MalwareBytes has a manual updating tool you can download,
For those computers which have been infected my malware, which takes over the Windows Explorer shell, run, command functions, Symantec released a tool which allows you to quickly and easily restore those functions.
And for home computers, I've found Microsofts relatively new Security Essentials free antivirus program to be very effective, it's free, light on resources, nag-free, and very effected in detection and cleaning. Simple to use interface.