As near as I can tell, many Android phones have a flashlight app that activates the LED flash, but there isn't an app that isn't specific to a given phone - someone has to figure out how to programmatically access the LED on each piece of hardware because there just isn't an API for that.
There's a patch for the Palm Pre that adds a menu item to turn on the LED, and I find it incredibly useful. Hoping someone can hack a similar thing for the Evo soon after I get mine, 'cause I'll definitely miss it.