In the Evans Gambit (1. e4 e5; 2. Nf3 Nc6; 3. Bc4 Bc5; 4. b4), is 4. ...Nxb4 a viable option for Black?  I've looked in a few opening books and a few online sources, and the possibility is not discussed.  White might be tempted to take the e5 pawn, but after 5. Nxe5 Qf6 the undefended Knight is pinned to the Rook and the Queen threatens immediate checkmate with 6. ...Qxf2++.  If 5. c3, then 5. ...Nc6; 6. d4 and the position is just like the common Evans Gambit variant 1. e4 e5; 2. Nf3 Nc6; 3. Bc4 Bc5; 4. b4 Bxb4; 5. c3 Bc5.

Here are two games with 4...Nxb4:

 

chessgames.com 

the reason Nxb4 isn't play is because you lose another tempo black has to move the knight after c3, and then the bishop after d4.

doesn't matter, i love Evan's Gambit, though white needs to make sure black doesn't get pieces out