I'm surprised that people don't seem to like this as much as I do. Yes, a designer is obviously going to use a more powerful tool, but that doesn't really say anything about how useful this is. I'm constantly writing small web apps and I have absolutely no design sense whatsoever. Having a really simple, elegant tool that lets me randomly choose a color and view a few shades lighter or darker is a huge help. It's got a nice, simple UI and does one thing very well. I'll definitely be using it.

And for those of you linking to other tools, they all either a) have a poor UI or b) do too much. That's not to say that they don't have their place, but they just don't do the same thing as this.