Discovering a new place to eat, meet nice people and have a good time is great. This happened to me yesterday at Carmencita Bar. Every Tuesday they have an Intercambio Night, where you can go and meet new people and practice languages. Beside that they offer brunch on Saturday (they say it is the best in town) and serve you delicious hamburgesas to a fair price. I found this place at Spotted by Locals, my first choice when I travel somewhere new in Europe.
Carmencita Bar is organized in the bar area with the kitchen and the restaurant where you can have dinner. It is close to Noviciado Metro and not far from the Malasaña barrio or Conde Duque, depending where you are heading that night, it is a perfect stop. On Tuesdays they offer their Mango Bellinis for 1€ and the host helps you to integrate into the groups of people and meet new friends.
Carmencita Bar opened in December 2010 and they have Intercambio Nights since September 2011. You should definitely check this place out on your next night out in Madrid.