Like Space Cowboy, I'm also a big fan of CCleaner. I tried out a whole bunch of file cleaners and Crap Cleaner is by far the best. I like it even more now that it cleans up Firefox cache, history, cookies and download history.
And thank you to Kenny94 for pointing out DG-Defragmenter. I was already using DIRMS and Find Junk Files, so this is an easy way to access both.
Here's a few more I find very handy:
Clipomatic - from Mike Lin, the guy who brought you Startup Control Panel.
Clipomatic is a clipboard cache program - it remembers what was copied to the clipboard even after new data is copied, and allows you to retrieve the old data.
http://mlin.net/Clipomatic.shtml
TinySpell - It is a small utility that allows you to easily and quickly check the spelling of words in any Windows application.
http://tinyspell.m6.net/
SceenPrint32 - A very versatile screen capture program.
ScreenPrint32 allows you to print and/or capture to disk or clipboard, the full desktop, active window, predefined area, or user selected area of the screen.
http://www.provtech.co.uk/software/screenprint32.asp