kuler is a rather cool Flash web app, from the fine folks at Adobe, that makes it simple to pick a pleasing color combination. Even better, if lets you save your combo and share it with the rest of the kuler users, who can rate the combos and find one that is just right for that Knight Rider tribute website they are working on.
But, I hear you asking, what the heck does this have to do with Macs? Well, my impatient reader, let me tell you about Mondrianum. Mondrianum is a cool plugin for your Mac that lets you access kuler combinations from within any app that has a color palette available. You can search kuler combos, see the highest rate, the most popular, or the most recent additions and you can even save favorite colors in little wells at the bottom of the window.
Mondrianum (which is a very clever name) is currently in beta and only works with Leopard.