This short video shows a very simple way to motorize a tripod track dolly using friction between two wheels. In the video below i've modded the Cinerails Snaptrack kit.
The main parts are a simple DC motor and Speed Controller from Servo City, and small venom 11V 3S Lipo battery. I mounted the Speed Controller up top so I don't need to bend over to change speed or direction. I mounted the motor just over one of the track wheels, and used washers to get the right amount of spacing.
Unfortunately these were spare parts laying around my house, so I don't have the exact part numbers. While I work on getting that together, here's a few links to get you started. I'm sure If you contact ServoCity.com they can probably help you out with an entire kit.
The Portable Track breaks down into individual pieces measuring 1m, and can be assembled to achieve over 16ft of run. Longer runs can be setup by purchasing additional track. The hard news is that this new track dolly system starts at over $680 dollars (found here).
Here's an inexpensive and lightweight way to add movement with your camera at different heights. YouTube member ImagineNowEntertnmnt used furniture sliders from his local Bed Bath & Beyond (as seen here) and placed it under each tripod leg [Thanks Justin]. The furniture sliders can glide along carpet, linoleum, ceramic tile, and cement, etc. With a mini tripod, I can see this being a handy tip for Wedding Videographers who need to travel light, but have access to lots of smooth surfaces to add slow camera movements.
There's a bit of shake in some examples which could possibly be corrected with a bit more weight hung on the tripod, or some post image stabilization, but in most cases it works quite well to add interesting camera movements to your footage. Pretty common stuff you can find at your local Bed Bath & Beyond, local hardware store, or even find available via Amazon (click here)