I'm black a lot and I really like being offensive, but turning it around on white can be really hard, so what's a good opening?
Please don't say fools mate or scholar's mate etc.
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This is an incredibly general question, because obviously white is allowed to make some moves as well … there is no one fits all aggressive defence. But to give an equally general answer: If you like to play aggressively playing gambits makes a lot of sense. So everything with "gambit" in its name is a valid choice.
I'll give a few examples and will assume that you didn't specify whites opening because all your opponents play 1.e4. ;-)
This would be the Latvian Gambit against the Italian Game: http://en.wikipedia.org/wiki/Latvian_Gambit
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 1.e4 e5 2.Nf3 f5
This is the Schliemann-Jaenisch-Gambit against the Ruy Lopez:
rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1 1.e4 e5 2.Nf3 Nc6 3.Bb5 f5
Generally the idea of these gambits is to speed up the development of your pieces at the cost of a pawn and to gain greater central control by diverting your opponents central pawns. If you play well, central control and active pieces will lead to an attack. If you play badly, you're just down a pawn ...
My kind of man! I love playing aggressive as black and I have found that the Sicilian works well. Its many variations can lead to passive or aggressive games. Until recently I favoured the dragon variation which fianchettoes you bishop in front of your king-side castled king. Now I have found a new line that I enjoy more. e4 c5, Nf3 e6, and if white plays d4 go d5. I love this counter attack because it can go badly for white either way he takes. It is called Marshall counter attack. e5 follows the develop Nc6. This will not happen all the time but it sets you up for a strong central positions with lots of modes of attack.
[FEN "rnbqkbnr/pp3ppp/4p3/2pp4/3PP3/5N2/PPP2PPP/RNBQKB1R w KQkq d6 0 4"]
Against e4, I like c5 (Sicilian Defense). Against d4, I like f5, (Dutch Defense), as suggested by another answerer.
These openings aren't totally offensive, because White has the offense. But they are counter-offensive.
Basically, they are as offensive as one can get while having the second move. It is a fight from the "get go" for the initiative.
If you want a recklessly aggressive defense, try the Saint George's defense (http://en.wikipedia.org/wiki/St._George_Defense). On a more serious note, the Sicilian Dragon or the Najdorf are solid aggressive defenses. Another aggressive defense that is overlooked is the Scandinavian. However, those are only good against e4 openings. Against d4 openings, the Dutch Leningrad is a very open, aggressive opening.
These are just opinions. My personal preferences differ from these though.
One resource you can try is looking up famous attacking chess masters, and checking out their favorite openings, for instance on the Most Played Openings section of their chessgames.com page. For instance, both Tal and Kasparov liked to play the Sicilian against 1. e4 and it shows up as their most-played opening as black.
You can see that a more all-round player like Spassky often played the Ruy Lopez, and that Karpov, who preferred closed positions and positional struggles, often played the Caro-Kann.
In general, though, even the attacking masters have to first equalize as black, then attack. If you get reckless you might fluster some opponents, especially in blitz, but you will also often be severely punished! Seeing how the attacking masters make that transition and go on the offensive is crucial if you want your attacks to really mean something and not just be superficially menacing-looking.