Check out Revolve. It is a Kickstarter project that has already reached its fund. I think it is one of the most versatile dolly around. Each corner of the dolly can be used to handle accessories.
And there are additional accessories (additional price, of course) to transform it even more. The best innovation (my opinion) is using the dolly wheels as a guide for the rail kit. Another is using it as a camera stabilizer. Check out the video.