iPhone users always had a thing for Flashlight apps. Way before the iPhone 4 came out with its LED flash, iPhone 3G and 3GS users used to install apps that turned the screen white and allowed them to see in the dark. Of course the iPhone 4 makes things simpler by allowing developers to access the LED Flash APIs and develop real Flashlight apps.
Too bad many of these apps are paid apps. Available for free in Cydia, SpringFlash is the first iPhone 4-only Cydia app that lets you turn the LED flash on / off with a simple Activator gesture. Mine is tap on volume heads up display.
Useful.