Cost: $1.95 each
Description: This basic tilt switch can easily be used to detect orientation. Inside the can are a pair of balls that make contact with the pins when the case is upright. Tilt the case over and the balls don't touch, thus not making a connection.
There are numerous uses for these basic sensors, but keep in mind you might need to use some debouncing code, as the sensor isn't immune to small vibrations and such.