Apply 5V and be shaken by this small, but powerful vibration motor. Works great as an physical indicator without notifying anyone but the wearer.

LilyPad is a wearable technology developed by Leah Buechley and cooperatively designed by Leah and SparkFun. Each LilyPad was creatively designed to have large connecting pads to allow them to be sewn into clothing. Various input, output, power, and sensor boards are available. They're even washable! (with care!)