Self-balancing scooters, like the Segway™ are often thought to be technological miracles, but it is not actually very hard to build one. I built the one described here in about a week using off-the-shelf parts. I spent another week tweaking the high-speed stability, improving the steering control, and writing about it.
Although the Segway has several exotic components, mine is built from common low-tech parts like wheelchair motors and RC car batteries. The parts, even at small quantity retail prices, cost less than half of a genuine Segway. It also doesn't need complex or high-performance software. The first version was written in Python and used serial ports to talk to the gyroscope and motor controller. The current software, now in C running in an onboard 8-bit microcontroller, is only 500 lines of code.