With Google's acquisition of Nest, you might be considering building your own Wi-Fi connected thermostat. Over on the Spark blog, they show you how to build your own open-source thermostat that connects to Wi-Fi so you can change the temperature from anywhere, and which logs your usage to the cloud.
While Spark's thermostat is by no means as intelligent as Nest's, it's still a cool project if you're looking to upgrade your thermostat. As you'd expect, they use a Spark Wi-Fi development board, ans LED, a Honeywell temperature sensor, and a motion sensor. The whole thing's enclosed in a nice wooden case, and it all connects to a server where all your thermostat data is logged. While it's certainly not as flashy as a Nest thermostat, it gets the basic job done to a point. The whole project is open source, and you can find the directions and software at the link below.