Here's a couple I can't live without
- Colorpad - It's a very light weight program that runs in a small window and allows you to easily select colours from anywhere on your screen. I use it all the time to get Hex codes from different sources and paste the code into the Inkscape colour palette.
- Screenshot Captor or WinSnap - Programs for grabbing screenshots. Handy for creating tutorials. Screenshot Captor has the advantage of being able to screenshot a scrolling window, but WinSnap handles windows with transparency better.