The 3pi is with $100 a relativly inexpensive robot, made by Pololu. It has:
- an ATmega168 microcontroller, which can be directly programmed in C or from the Arduino Environment
- two real wheels with micro metal gearmotors, driven by drivven by real motor drivers
- five reflectance sensors
- an 8×2 character LCD
- a buzzer
- three user pushbuttons
It comes fully assembled, but it can be extended by some PCB cutouts.