zlacker

I second the Hue recommendation. They're a solid product. They aren't as good at rendering arbitrary color, but that's because they optimized for excellent whites.

Their API is also really good: clear, well-documented, REST-y. I wrote my own software to simulate a day/night cycle in my house, and I'm very happy with it. It's a much easier way to wake up, and having the gradual dimming in the evening makes it easier to go to bed on time.