The Phunky Philidor
It's a perfectly solid opening. It's perhaps slightly cramped, but it's not unsound at all. It also has the boon of taking most White players out of far more familiar territory, so there's a practical advantage to playing it.

Philidor is seen as reliable since the good move order has been found :
1.e4 d6 2.d4 Nf6 3.Nc3 e5, which reaches the "classical" philidor after 4.Nf3 Nbd7 5.Bc4 Be7...
Of course white has options, like 4.dxe5 and Shirov gambit with 5.g4, but that's another story and theorically speaking, black seems ok in either case.
U can find top GM games on Philidor, even Carlsen played it.
Moreover, exchange philidor with 1.e4 e5 2.Nf3 d6 3.d4 exd4 followed by Nf6 and Be7 is playable.
Like mecuelgalapieza said, fashion...
About the move orders...
- The historical one : 1.e4 e5 2.Nf3 d6 3.d4 Nbd7 just brings white nice advantage after 4.Bc4 c6 (forced for tactical reasons) 5.0-0 Be7 6.dxe5! dxe5 7.Ng5 (the point) Bxg5 8.Qh5 threatening f7 and g5, so black has to protect f7 and white will recover his piece with bishop pair.
- The lion move order : 1.e4 d6 2.d4 Nf6 3.Nc3 Nbd7 has problems after either 4.f4 or 4.g4 (i prefer g4, simpler...) .
- Another move order is 1.e4 e5 2.Nf3 d6 3.d4 Nf6 but fails to 4.dxe5 Nxe4 6.Qd5 Nc5 7.Bg5 with once again a nice advantage for white.