A very useful function of my home automation system is that there's a little light inside my house, on a light switch, that tells me whether or not my garage door is open. This is useful because the light is on the second floor and the garage door is on the ground floor, and not having to walk down two flights of stairs every night before bed is rather handy. Instructables user Steve Moseley shows you how to make this without having a home automation system.
The short of it is that you need an LED for displaying your garage door's status, a switch that detects whether or not the door's open, and enough materials to mount both in place. If you wanted to get fancy, you could add some sort of wireless system so you can have this switch in any part of the house as well, though the instructables doesn't cover that.
Garage Door Open Indicator Light | Instructables