ronmatt, Gary Eden, and our other members who use OS X might find this valuable, and it's free.doodim
dims the desktop background so your foreground app can stand out. I'd think this is good for accurately evaluating colors as you balance images without the distractions.
BTW, if you want to dim the document background in Photoshop, drag the document borders out so you can see the background, pick a foreground color, and then with the Paint Bucket tool, Shift+click. This is not an action, so it's not undoable--so think twice before choosing chartreuse!