Ummm... at least on my Aria (unlocked version) there is no LED light on the phone. If there is no light, then how would an app be able to turn on the (non-existent) LED?
If your phone has LED flash for taking photos, there are apps that can use that same light as a flashlight (e.g. TeslaLED). For phones with no LED, the best you can get are apps that make the screen all white.