If your thermostat just stopped working you could go to the store and purchase a new one but why not make your own. You can add any feature you like, want to control it from your smart phone? Add an ethernet module and go for it. Want to add schedules? Add a RTC and you can program timed temperature settings to your hearts content. Want to make sure the average temperature in your house is kept to a certain temperature? Add multiple sensors in various areas and average them together or add some dampers to control the air flow in various zones. Your imagination if the only limitation.
This Arduino Thermostat Project by Dylon124 is just the basics but from this starting place anything can be done.