A while back Jason made a phone controlled rover using the MotorAir. He wanted to revisit the basic idea but using cheaper, more widely available parts. Also since this is Arduino based, it is a great springboard for more than just a rover that drives around. You could add sensors, servos, etc. to really drive this project in any direction you want.
https://youtu.be/Qx0tvpr2DNw
Get started building your own with Jason's easy step-by-step instrutctable!