In principle, this breakbeam sensor could be adapted to a lot of other things - an alarm or bell when someone steps through a doorway, turning the lights on when someone steps in a room, and so on. I've provided a simple sketch, but there's a lot of room for improvement - using the AVR "watchdog" library to automatically reset the processor if it hangs, or using the low-power modes so that the whole thing can run off of a battery instead of the wall. The great thing about the Arduino platform is that you've got a lot of options and it's easy to explore them. There's a lot of tools to help. If this is your first electronics project, congratulations! You've made something useful. Check out Fritzing for examples of other circuits you can build.