Another option is the Parallax Propeller which is the microcontroller, sort of like the Arduinos use Atmel microcontrollers. Here's a sample of a robot controller made from a Parallax Propeller.
http://mikronauts.com/roboprop/
These Parallax Propeller microcontrollers are extremely powerful and support simultaneous operations in their cog infrastructure.
The can be programmed in spin (assembler), pBASIC (like the BASIC Stamp) and C.
The 40 pin DIP chips themselves are basically $15 each.
The only real draw back is that the Propeller is natively 3.3V. So if you plan on interfacing lots of custom 5V logic be prepared to use 2N7000s or level translators (SparkFun sells premade level converter modules).