Speederaser talks to any Android cellphone. I suggest this one, but any will work.
“Advanced Cruise Control” tells your cruise control the speed limit.
1. Following this picture, buy this board, these resistors. wire it into the clockspring.
2. Get the app and sourcecode.
More discussion places:
Un-maintained version on makezine.
Due to popular demand, we are now selling one at cost: Buy it here.