Sparmatic is a thermostat that is relatively cheap and quite easy to mod. It costs from 13 to 25 EUR. It contains a simple AVR ATmega169/ATmega169PA. However, as there are different microcontrollers mentioned in the last sentence, there are several revisions of the device.
We are currently writing an alternative Firmware for one one of these revisions. You can get it by cloning this git repository:
git clone http://www.0x53a.de/git/sparmatic