You the Man
By New York City Department of Transportation
Open iTunes to buy and download apps.
You the Man is a handy tool that makes it easy to calculate your Blood Alcohol Content, along with a some other handy features. Stayed out a little long and want to call a cab? You the Man's got you covered — you can be on the phone with the nearest car service with just two taps. Play Spin the Bottle to pick tonight's designated driver, and send a shout out on Twitter to thank your responsible friend!
Best of all, it's absolutely free, and works on an iPod touch if that the sort of thing you're into.