I used to live in a 3rd floor apartment with an old water heating system, which meant that unless I kept careful control of the thermostat, my apartment would get really hot in the winter. I designed an Arduino-based system to automate my home's thermostat by using a servo to manually set the temperature knob.
The goal of this project was to be as low budget, low development time as possible -- I made it in about a week. Even so, it ran all winter and until I left that apartment. This was a personal project, but I later put together a detailed writeup on the design for a school assignment.