Color synthAxis is one of those interactive color choosing tools you see every once in while on the web. But this one is cooler than most, providing a powerful range of tools to manipulate colors in different ways. I'd call it a color processor, the way Photoshop is an image processor.
You have a large palette full of swatches. Each one can be clicked to expose a set of tiny tools around it, which you use to combine, split, mix, and match with other swatches.
I'm not a color person (can't you tell from my design for this site?), but even I was mesmerized by the different shades I could create with this thing.