If you've never heard of RGB or HEX codes then ColorConverter won't be of much interest or use to you. But if you're a designer, you'll know how important they are.
ColorConverter is a free tool that gives you free access to the MacOS X color picker, plus the RGB and HEX codes for any color you choose. For fine tuning, there's a magnifying glass feature, allowing you to target a specific color. Once you've found the code you need, just hit Cmd+C to copy it.
There's little more to ColorConverter than that. The only drawback I can imagine is that any decent design tool should have this function already built-in, so for serious designers it's hardly essential but for those that are lacking it, it's a handy free tool.
ColorConverter will be a welcome addition to any designer's toolbox that needs RGB and HEX codes instantly.