Cost: $35 - $120
Time Spent: 1 Hour
Tools: Screwdriver, Batteries, Programmable Thermostat
Installing my Honeywell 5+2 Thermostat a year ago was one of the best and easiest projects I've done for my home. It was one of the first things I did after moving into my place and it paid off within the year. Using a programmable thermostat eliminates the need to remember turn the temperature up (or down) and saves energy and money.
Honeywell has plenty of good programmable thermostat options - we choose the 5+2 because it gives use just enough flexibility to have one schedule for the week, and another for the weekend. There are other options such as a 7-day programmable thermostat that allows for a different temperature schedule for each day, but the added flexibility also results in a more expensive thermostat.
Installation depends on your exisiting HVAC setup, but This Old House has a nice demonstration video. Also, if you have the option, the location of your thermostat can affect its performance and efficiency. Always locate thermostats on interior walls and away from direct sunlight, exterior doors and windows.