Reed Switch Hookup Guide
Resources and Going Further
Reed switches are a perfect trick for non-contact activation applications. We used a reed switch to detect when the doors of this TARDIS-themed music box opened. Or you can hook a large, door-specific reed switch up to a Blynk Board and have it send push notifications to your phone when the door state changes.
Now that you've got a reed switch wired up to your Arduino, what are you going to create? Need some inspiration, check out some of these related SparkFun tutorials: