It's nearly pointless to know who's running happy hours without also knowing what kind of booze is on special, as you're left with very little justification for your chocolatini orders on $3 whiskey night. For an app that breaks down specials drank by drank, check out DrinkOwl.
A just-launched site/iPhone app
that shows you where to capture and imbibe those delicious nocturnal creatures, OW cleanly and comprehensively shows you what spots have what specials when, elevating itself above the competition by allowing you to filter drinks, a technique originally mastered by college juniors with Popov and a Brita. Geolocate yourself to bring up a Gmap stocked full of all nearby watering holes running specials (or on the web, just scan a map of all of DC); you can then filter by day of the week, and each spot's marked with a color indicating whether they're discounting beer, wine, liquor, other, or "mix", which better not be full price, now that 107.3's without the peppy banter of Tommy McFly. They've already got over 100 DC bars on there (from good ole "assorted pints" at Lucky Bar, to wine deals at Hudson), and will also prioritize venues based on their proximity to you in a feature called the "List", which ironically, will help inhibit you from getting to anything on tomorrow's "to do" one.
Soon, Owl users'll be able to create a "favorites" page for easy scheduling, and show their top specials via social networks to all of their friends -- though given your bar orders, the same could also be done by saying "Hey Tim, come look at my computer".