Hues offers an easy way to get and convert colors. Hues shows the color in Hex, RGB(a), and HSL(a). You need to grab a color from an image, make it a little bit darker, and get the new hex value. You don't want to have to open a design program just to do that. With Hues, you'll be done before Photoshop has finished opening.
A fast, lightweight color picker app allows you to grab the color of any pixel on your screen. Built on top of the standard Mac OS X color picker that you're already familiar with. Whether you're a designer or developer, if you work with colors, you'll want Hues.