Instructables user “stoppi71” has been building a DIY Segway for a while now, and just posted a bunch of info on the project.
Balancing on two side-by-side wheels is rather difficult, though as the original Segway showed us, it’s quite possible with electronics to help. Naturally, hobbyists have tried to duplicate this effort, including stoppi71, who started his experiments several years ago. He uses both an accelerometer and gyroscope to determine the angle, along with PID control to apply the correct amount of power to each wheel. Buttons on either side handle steering.
Though not the easiest project featured here, if you’re thinking about doing something simlar, his writeup is worth a look! If you enjoy alternative modes of transportation, you may want to check out this electric unicycle as well!