In the middle of a night of heavy drinking is probably the worst time to judge if you've had too much drink to drive, or maybe even walk. Buzz Buddy for iPhone and iPod Touch is a good stand-in for your conscience--if you can remember to use it.
After entering your gender, weight, and whether you've already consumed alcohol, you'll tap the drink type you're about to guzzle. Buzz Buddy will keep track of your beer, wine, cocktail, and shots count. Using your weight and gender, it'll graph your estimated blood alcohol level over the hours, helping you predict when your levels will get back into the safety range.
If you've spiked into the danger zone, there's a button you can tap to call a cab. The colored indicator is supposed to shift shades as you become more intoxicated, but it stayed green for us even as we "chugged" enough booze to fell a mule.
Buzz Buddy is a good idea otherwise--just as long as you have the presence of mind to use it--but there is room for growth. Dialing a bailout buddy from your contact list is one idea; a map that finds late-night munching spots nearby is another. We'd even settle for the ability to manually enter drinks on the graph that you may have imbibed earlier but had forgotten to add, so you can calculate your current levels even after your night is over.