Albin Countergambit
The Albin starts as a Queen's Gambit. But then Black sacrifices a pawn on move 2 and pulls White out of book almost immediately. He does this to gain rapid development, place a strong pawn on d4, and create weak spots to attack.
This is generally considered to be dubious. However, it is potentially lethal - especially to a player who has not seen the Albin. If White just uses common sense, he may end up just fine. He may also be burned. The latter commonly happens through the Lasker Trap, which involves an underpromotion on move 7 (!)
Even if White does not take the Knight on g1 right away, Black will usually follow up with Qh4+, bring out the light-squared Bishop and get a very strong game.
With correct play, White can achieve a more comfortable position. But it is still sharp. The opposite-side castling and Black's pawn on d4 can easily play in Black's favor, and h7-h5-h4 along with Bh3 will often give him a very strong attack. However, the extra Pawn which White posesses plays out in his favor in most endgames, and the potential a3 and b4-b5 will give him good chances.
The Albin is not the soundest opening, but it is extremely aggressive. I would reccomend it to anyone who likes to play aggressively and hates to play against 1. d4.