OK, so people are making fun of the unplug notification. It is practical. On my MacBook Pro, I would actually like to know if the MagSafe plug has disconnected, because there are times when the first time I noticed that the power lead was disconnected is because a low battery warning popped up...and I needed that battery to stay charged for when I left the building.
But I also use Hardware Growler to let me know about things like when the wireless router I'm using (away from home) goes offline, or to confirm when a USB or FireWire volume is fully dismounted so that I can unplug it. There are more examples but suffice to say I have found Hardware Growler informative for years with Growl. It can also be used for things like reporting Bluetooth devices coming into range.