As someone who never understood the position nuances of opening play, most of my opening theory has been aimed towards forcing lines where I get some sort of material gain/mating attack. I am quite partial to lines involving gambits and sacrifices but again this is geared towards mating attacks (eg the Traxler, Muzio or BDM).
However I am impressed by early sacrifices which don't lead directly to mate, but the slightly more subtle 'initiative'. Probably the most obvious (and unsound) example of this is the Halloween Gambit (1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Nxe5??!) where White does get to push the Black knights around for a while.
While arbiting Street Chess today I saw a game involving similar ideas, but in this case it was entirely sound. White decided to offer a piece for threats on the e file, and black decided to return the piece as a way of dealing with them. Unfortunately this allowed White to set up a mating threat which surprisingly Black missed.
Fitzpatrick,Brian - Yuan,Yi [C80]
Street Chess , 12.12.2009
1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.0-0 Nxe4 6.Re1 Nc5 7.Nc3 Nxa4 8.Nxe5 Be7 (D)
9.Nd5 0-0 10.Nxc6 dxc6 11.Nxe7+ Kh8 12.Qh5 Nb6 13.Re4 Nd5 14.Qxh7+ 1-0