This is a two-for-one.
If you do any kind of design or development, you’re constantly needing to match colors (or you should be).
Nattyware’s Pixie and Apple’s Digital Color MeterÂ (if you’re using OS X Lion, which you should be, be sure to see the new way to get Hex values) are what I rely on. Both free. Both easy to use. Launch, roll over the color you want to match, and you get your RGB, Hex and other values.
I use these almost daily for web design and elearning development.