Diemer-Duhm Gambit: ECO D30b
A sharp and entertaining line to play against the French Defense or, less often, the Queen's Gambit Declined.
1. e4 e6 2. d4 d5 3. c4! ...
The main idea here is much like any other gambit: Sacrificing material for a tempo and a lead in development. In this line, even the declined variations tend to let white tear open the center and start a vicious attack.
This is particularly fun against the French Defense, since French players tend to like slow and/or closed positional games, and are usually unhappy to see the center gone in the first few moves. Ripping open the center leads to a fast-paced tactical game. This is not an opening for the faint of heart.
The accepted variation (3. ... dxe4 4. f3 exf3 5. Nxf3 ...) leads white to a violent kingside attack, many times crushing the opposing king before black can get all of his pieces in order. If you're a strong attacking player who's not afraid of storming the enemy king in his home, you should consider adding the DDG to your repetoire.
Named by Emil Joseph Diemer (Of Blackmar-Diemer Gambit fame) and based on a correspondence game in 1908-1909 by Andreas Duhm, the DDG has gained a great following by gambit players in recent years, mostly thanks to the advocacy of its play by Jyrki Heikkinen of Finland.