haptiClock vibrates you the time (haptic feedback) if you shake/bump/tap your phone. It works when your screen is off, doesn't use much battery. The pattern to decipher the time from the pulses is simple to learn.
Great for checking the time on the sly, or when you can't pull your phone out.
Free version for trial then limited, and then paid for full version. Send me feedback, I want to make this a useful app!
[EDIT: Some (mostly HTC with Sense UI) phones app is not working in standby mode... rendering app not too useful. I am working on it ]