Last January, we had some trouble with the heat in my office. Specifically, the kind of trouble wherein the heat is not on, you turn it up, and it still not on. This went on for more than a few days, and finally ended a day or two after we got an email announcing that the heat was broken and speculating that it had probably been down for a few days. My teammates and I laughed a bit at this - we knew exactly when the heat had stopped working. We had a continuous record of the temperature in the office going back months, with 10-minute resolution.
You can do this too, and it's quick, cheap, and easy! This little gadget is built around an electric imp, and you can push the data from the imp out to anywhere you want. In my case, I found it handy to push the data to Sparkfun's Data Stream service, which stores the data for free. Even better, I can use imp.guru's Sparkfun Data Stream frontend to get nice graphs of the data I've collected.
This project takes about an hour to two hours, if you've never done a project with an electric imp before, and when you're done you'll have a thermometer that you can toss anywhere with WiFi and collect data for months to years on a single battery, depending on how often you check the temperature.