A square in Prenzlauer Berg district just north of the perhaps better known Kollwitzplatz. The square is dotted with bars and cafes which continue off along Raumer-, Lychener-, and Dunckerstraße (where I live as it happens). It's a great summer hang out. Plus there's a youth hostel on the North side of the square.
Nearest stations are Schoenhauser Allee and Prenzlauer Alle (both S-Bahn), and Eberswalder Straße (U-Bahn)