The parts system being used for the FTC competition (besides the NXT), TETRIX, is available to the public on Pitsco's website, here. Although the parts are pretty expensive, this system basically allows you to take your NXT to the next level. 12-volt DC motors and strong aluminum beams and other parts let you build big robots that are still controlled by the NXT Brick. With the motor controllers that allow the NXT to control the non-LEGO motors, you can have many more than three motors; by stringing motor controllers together, you can have almost as many motors as you want, provided you have enough batteries.
One important problem with the Pitsco store is that it doesn't seem to sell the HiTechnic controllers which are required to let the NXT interface with the TETRIX motors. Hopefully these will be available from HiTechnic soon.