Popular models are:
-sabertooth either 15 or 30A from dimension engineering (in America). Each one controls 2 motors. People say you shouldn't push them to their limits or they overheat and can be fradgile
-electronize either 15 or 30A I think, they are cheaper and more robust but you need 2 and they are quite large. They don't give very precise control.
-some people (razordave I think) use victors to control more powerful motors like speed 900s
- I have also heard of sidewinders which I know nitging about but I think there are some in the for sale section.

Edit: other people type faster!