ChargerChecker for Mac

From Chad Brantly:

ChargerChecker is the best, most full-featured power state alert app available. It works in conjunction with Growl to give you on-screen notifications when your laptop’s charger is connected or disconnected. With the release of the MacBook Pro, Apple also introduced a brand new power adapter that uses the MagSafe connector. While this innovative new feature can save your laptop from being jerked off the table by a careless passer-by, many people have complained that their power adapter becomes unplugged inadvertently without them ever knowing it. This can be a huge problem when you pull your laptop out in the airport to put the finishing touches on your upcoming presentation only to find that it was mistakingly unplugged and the battery is completely dead. Install ChargerChecker and never worry about a discharged battery again.