When I was a kid, I was quite active in the soda race forums and the community there. In fact, my first open source project was a library for generating soda constructor models programmatically.

For example, it let you write text: http://sodaplay.com/creators/russellsprouts/items/font

Some good creators, I'm sure there are many more I'm forgetting:

http://sodaplay.com/creators/mmaarrkkuuss

http://sodaplay.com/creators/edlinfan

http://sodaplay.com/creators/grape

http://sodaplay.com/creators/jeckyll

http://sodaplay.com/creators/im_an_alien

http://sodaplay.com/creators/phider

http://sodaplay.com/creators/lectvay

http://warptera.com/

Many models are made up of motors with legs attached. Here are some interesting ones:

http://sodaplay.com/creators/motors

http://sodaplay.com/creators/other_motors

Also of interest, this paper empirically discovers the physics behind it: http://falcoprof.altervista.org/dispense/sodamathspaperfull....