If you don’t own a Samsung Galaxy Nexus or HTC Nexus One, move along. I guess this app works for other phones in a limited capacity, but from what I can tell I would just as well skip it. However, if you have one of the two mentioned phones, this app is a must.
Light Flow lets you completely customize your build in multi-color LED for notifications. What it does is make sure that the LED blinks a specified color for any notification that your phone receives, that way you can tell exactly what kind of message is waiting for you even with the screen off. It supports all the built in apps like Voice Mail, Messaging, and Gmail, as well as third party apps like Foursquare and Tweetdeck. It can also activate the LED for low battery, no signal, bluetooth connectivity, etc.
This functionality should really just be built in, and is on some custom roms. To me, it is indispensable. You can pick it up in the Google Play Store for $1.99. There is also a "Lite" version that doesn’t have as much third-party app support.