For some reason, this line seems to be currently in fashion on FICS (or at least, I keep running into it). I normally just castle, exchange my bishop if it's challenged by a later ...a6, and try to go for the center with my pawns, usually with the kingside rook supporting the e pawn. Does anyone know of a nice sharp tactical line I could use to counter Black's ...Qf6? I don't see any obvious weaknesses, but the move seems wrong on principle.

4.Nc3  Nge7  5.d3  Nd4  6.Nxd4  exd4  7.Ne2  c6 (if ...a6  8.Bc4)

Try this:

1.e4 e5 2.Nf3 Nc6 3.Bb5 Qf6 4.O-O Bc5 5.Nc3 Nge7 6.Nd5 Nxd5 7.exd5 Nd4 8.Nxd4 Bxd4 9.c3 Bb6 10.d4 exd4 11.Re1

I love to play this variation, offtherook! :)

Searching through my old books, in Mikhail Yudovich's "Spanish without ... a6", he gives the line given by kco in post 2. He gives up to move 9. 4. Nc3 Nge7 5. d3 Nd4 (5... h6 is more popular) 6. Nxd4 exd4  7. Ne2 c6 8. Ba4 d5 9. 0-0 with an advantage. Schallopp-Berger, Frankfurt 1887. However, my database gives a game Bogoljubow - E. Lasker, New York 1924 where Black plays 9... g6 and after 57 1-0. So I don't think there is much in the sharp tactical lines here. As with most Lopezes, you may have to grind Black down.

3...Qf6 is a pretty bad move.  Developing normally should give you a good advantage because he'll have to move his queen again sometime soon and his king's knight is denied its best square.  The e4 pawn isn't even being attacked!  After 1.e4 e5 2.Nf3 Nc6 3.Bb5 Qf6, probably c3 is best, preventing the irksome ...Nd4, then 0-0, then play a normal ruy against an opponent whose pieces are not coordinated.

I could see playing BxNc6 if you could capture his e-pawn.  Otherwise I'd prefer to keep the bishop.  After he recaptures with the d-pawn you'll never get to use d5 and the exchange ruy, which isn't that bad for black anyway, is probably better for him with queens on and castling rights.  His position still sucks but keeping the bishop pair and avoiding exchanges to keep him bottled up while he figures out what to do with his Q and N has got to be pretty good too.

If he's bad enough to play ...Qf6 he might let you trap his queen later but otherwise I don't see any quick strike.  His king still has plenty of cover.

Most databases show 4.Nc3 or 4.O-O as pretty good for white. If you want to get into tactics quick, simply 4.d4 looks good. The problem with 3...Qf6 is that it takes away a good square for the g8 knight. Black can't develop normally. There's also no good square for the f8 bishop. After 4.d4, black needs an extra move to get his c8 bishop out to enact the pin on g4. After 4...d6 or d5, the knight is self-pinned on c6 so that 5.d5 or exd6 is a headache for black. If 4...exd4, simply 5.O-O, threatening e5. So black's best seems to be.....