Albin Counter-Gambit
1.d4 d5 2.c4 e5
The Albin Counter-Gambit can take the unprepared White player by surprise. If White takes the e pawn, the black d pawn moves to d4 and can be an effective wedge splitting the White position into two later. It also maintains a threat of d3 which can worry the White player. Additionally, Black can take a very agressive approach and play moves such as Be6 Qd7, Nc6 and castle queenside with the idea of going for White's king on the kingside. The gambit therefore can give the black player attacking opportunities, and White therefore needs to have a comfortable system prepared against this.