Autosafe Drinking lets you know exactly when you're fit to drive after drinking, based on accurate calculations, but it takes a little while to figure out. You'll definitely want to have it set up before you enter the bar instead of trying to fiddle with it after you've had one too many. While it's not a replacement for a designated driver by any means, it's a good way to figure out exactly how alcohol impairs your motor skills and thinking.
This app asks you for your body type and what you've had to drink to calculate your blood alcohol level. It's fairly accurate, but you probably want to take any of its recommendations with a grain of salt and hand your keys off. The app has calculations for just about every style of beer, spirit, or mixed drink and lets you adjust the measurements to metric or standard. You can change your own measurements to be in a numeric system you understand, as well. You can also add your favorite drinks to the list, which is quite convenient. In addition to throwing a color-coded BAC estimate your way, Autosafe Drinking also gives you fun facts about alcohol and drinking, which you can easily share online with your friends. The app even has a timer that gives you an idea of when the alcohol will have run its course.
It's an incredibly well conceived app and it works offline for those rare bars that don't offer free Wi-Fi. Though it's probably never a great idea to get behind the wheel after a night of drinking, Autosafe Drinking can give you a good idea of when you should pack it in for the night. Anything's better than that embarrassing "I think you've had enough" from the friendly barkeep, right?