Van Geet's Opening (also known as the Dunst Opening), can often be a useful surprise weapon. The opening beings with 1.Nc3 and basically invites Black to choose what happens next. Sometimes Black will choose lines that transpose into other known openings (eg 1... c5 heading for a Closed Sicilian) but normally the reflexive 1... d5 or 1... e5 is played before Black even thinks of what to do next. This provides White with an opportunity to lead Black down some dangerous lines.
I first became aware of the danger the opening poses for Black in an article by IM Gary Lane in Australian Chess Forum. Since then I've found it a useful addition to my repertoire, especially against booked up juniors. It's even appeared in the higher chess echelons with the top two boards of the Papua New Guinea Olympiad team using it (with varying degrees of success) in the 2006 Turin Olympiad.
To show you how badly it can turn out for Black, here is a very short game from 2001. NB This is a plausible line for Black (at least until the end) as I found a couple of versions of this game in my database.
Havenaar,J (2235) - Von Saldern,R (2133) [A00]
Guernsey op 27th Guernsey (5), 25.10.2001
1.Nc3 d5 2.e4 dxe4 3.Nxe4 e5 4.Bc4 Bf5 5.Qf3 Bg6 6.Ng5 Nf6 7.Qb3 1-0
My main use for the opening (apart from shock value at Olympiads) is when I sit down at the board with no idea what opening I should play. Here is a game I played last week that demonstrates this.
1.Nc3 d5 2.e4 d4 3.Nce2 c5 4.f4 This is now a line of the Sicilian Grand Prix, although still classified A00 by Chessbase. 4...Nc6 5.Nf3 Nf6 6.Ng3 e5? The start of an unsound sacrificial plan. 7.fxe5 Nxe5 [7...Ng4 8.Bb5+=] 8.Nxe5 Bd6 9.Bb5+ Kf8 10.Nf3 Bxg3+ 11.hxg3 Nxe4 12.0-0 Bg4? [12...Nxg3 had to be played to justify the preceding moves.] 13.Re1 Nd6 14.Be2 h5 15.d3 f6 16.Nh4 Bxe2 17.Qxe2 Kg8 18.Ng6 Rh7 19.Qe6+ Nf7 20.Bf4! Black is now Zugzwanged! 20...Rc8 21.Ne7+ Kf8 22.Nxc8 1-0
Of course no opening is a forced win, but if you want to avoid the perils of memorisation, 1.Nc3 is a good choice. However, one final warning. In a recent article in New In Chess it was observed that Dirk Van Geet, of whom the opening is named after, gave up playing it long ago.