These are the projects I've been using as examples but there are lots of them and some may be better, so don't stop searching for them.
[Vertibot] (https://madebyfrutos.wordpress.com/2013/05/02/vertibot/)
[Balanduino] ( http://blog.tkjelectronics.dk/2012/03/the-balancing-robot/)
[ArduRoller] (http://makezine.com/projects/arduroller-self-balancing-robot/)
