Here is an interesting gadget – HOBO® U10 Data Loggers. It may record temperature or temperature and relative humidity in indoor environments. The logger has 52K built-in memory to store data and direct USB interface to transfer it to the PC. Using collected information and included HOBOware® software you’ll be able to build time-stamped graphs. From this analysis, you can tell whether the heat is running whenever you’re out in order to change the thermostat temperature to lower costs.
The logger uses CR-2032 lithium battery to get energy (1-year battery life typical).
You can buy HOBO® U10 Data Loggers with temperature sensor for $55 and with temperature/humidity for $69.