dict.leo.org -- An online dictionary that is really good at helping you decide between two words of similar meaning.
www.codersnotes.com/sleepy -- very sleepy is a simple sampling CPU profiler with call-graph. Although it slows down the application quite a lot, just starting it and doing some sampling is so quick and convenient, that I usually first try this little hack before starting up a "real" profiler.
www.farbrausch.com/prod.py?which=182 -- Planpad is a very fast local wiki. I do all my todo-lists and planning with it. No wonder I love it, I programmed it...
www.wolframalpha.com -- Sometimes you can solve math problems by just entering them into WolframAlpha. It's not as good as the "real thing", but I don't have Mathematica.
Microsoft Paint -- I actually am a pro user of this tool! I know all the keyboard shortcuts and tricks and stuff. If you just need to printscreen, crop and save screenshots, then it's quite good. Unfortunately it uses ClearType with the text tool, which is quite stupid because the image will look wrong on a different monitor :-(