The Irobot Create Command Module is a add-on component that attaches to the robot via the 25-pin serial port in the Create's cargo bay. It contains an Atmel AVR ATMega168 microcontroller that can allow interfacing with the robot hardware. Effectively, rather than directly writing out commands over a serial port, you can write simple code in either C or C++ using AVR editing programs that act as a layer of abstraction between the programmer and the ROI.
The command module also has four 9-pin I/O ports which allow you to connect hardware. It comes with several example programs (a drive program, a wall-follow program, and a program which demonstrates how to add hardware (light sensor and a button) to create a program that searches out light sources).
Downside: the layer of abstraction is often very thin and still requires being very careful about passing around particular byte codes, storing things in particular memory registers, etc.
Still requires a lot of "heavy lifting".
Example Code: 
Link to Command Module User Manual: