The Viennese shock
I believe that the Vienna game is one of the most underrated openings that the white pieces have to offer, in particular the Vienna gambit. This opening has shocked many an opponent in rapid and lightning games I have played, sometimes they take over 15 seconds of their original minute to think of their response.
You see, most people expect 2.Nf3 of 2.Bc4 after 1.e4 e5, but for some reason, when you play 2.Nc3 it seems to catch people unaware, taking them into an unknown opening territory in most cases. Most of the time they reply with 2...Nf6, with which 3.f4 totally throws them into the deep end.
There are two main responses to this move. The book move is 3...d5, which after 4.fxe5 Nxe4 offers good chances for both colours:
Most people, however reply with 3...exf4, to which 4e5! sends the knight back to g8 (4...Qe7 5.Qe2 Ng8 transposes), and gives white a very good game.This is a very handy opening to have up your sleeves, especially in quickplay games, where your opponents don't have as much time to think about each individual move.