Make:

HomeToys has a pretty intense garage door monitoring project (code included) - "One critical monitoring element for any home automation system is the garage door. There are many ways to do this including using your traditional magnetic contacts which will change state when the garage door is open or closed. This method satisfies the minimal requirement of knowing the open/close status of the garage door. But what happens if more detail about the garage door is needed? For instance what can you do if you would like to know the exact position of the door (say if it got stuck during its open/close travel)? What if you wanted to know this garage door position to within a few inches?" [via] - Link.