This A-Star can be used as a Raspberry Pi HAT (for Model B+, Model A+, and Pi 2 Model B Pi boards) or as a standalone controller for a very simple robot. Built around an ATmega32U4 AVR microcontroller with built-in USB functionality, the board includes dual motor drivers that can provide a continuous 1.8 A per channel. It also has pushbuttons, LEDs, and a buzzer and ships with a preloaded Arduino-compatible bootloader.
If used in tandem with a Raspberry Pi, let the Pi handle the high level tasks while the board takes care of simpler things, such as reading analog sensors and controlling servos.
The A-Star 32U4 Robot Controller LV with Raspberry Pi Bridge can be purchased from Pololu for $32.95.