This Dashboard widget post (see the others) is more for developers and designers than the average person.
Colourmod is a Dashboard widget for Mac OS X that you'll end up using much more than you think you would. I'm not a designer but as someone who fiddles around on the web I find myself in need of ahexadecimalcolor code from time-to-time. And when I do, I use Colourmod.
There are a few ways that I find myself using Colourmod to find the color I'm looking for. The first, and perhaps most obvious way, is to drag the "blue dot" color picker around the main color well and find the color I want making subtle adjustments by using the slider. The second, is to manually enter in the hexadecimal color code that I'm currently tweaking and make small changes until I get exactly what I want.
One nice feature is the blue arrow that will quickly copy the current color code and place that value into your clipboard. This makes finding, selecting and copying a color code into your text editor very, very quick.
Feature suggestions? Sure. One thing I'd like to see is a single text area that gives the proper RGB color values for a color. Although Colourmod supplies these values they aren't easily selected. I'd also like to see a much easier and more accurate way to use the color picker. It is very tough to make small changes especially to light gray colors.
Oh, and I'd ditch the 'U' in Colourmod. But that's just me.