Don't waste your time trying to talk someone up at the bar for a free drink -- some bars are "following" the new #trend of advertising through social media.
Gabriela's Tequila Bar:
Every third check-in on foursquare with get you a free frozen margarita and they have a selection of about 80 tequilas - not bad. Make an average Tuesday like it's Cinco de Mayo.
Check-in to this French resturant on foursquare you will get a free cocktail. There's also free jazz performances at this Upper East Side venue.
Le Poisson Rouge:
A first time check-in on foursquare will get you a free drink after you buy the first one. The space also has a gallery and big name bands play at it's concert venue.