Why would one promote a pawn to a knight? I know that it is makes no sense to promote a pawn to a bishop or a rook as the queen can compensate for both but a knight is something different. However, the queen is still much more powerful. So what situations would provoke you into promoting your hard-labored pawn to just a knight rather than a queen?
White to move:

[FEN "8/q1P1k3/8/8/8/8/6PP/7K w - - 0 1"]


Since my example is rather contrived and artificial, I'll also say that the so-called Lasker trap in the Albin Countergambit gives a more realistic setting, and one where a knight promotion is the best option as early as move 7:

[FEN ""]

1.d4 d5 2.c4 e5 3.dxe5 d4 4.e3 $2 Bb4+ 5.Bd2 dxe3! 6.Bxb4 $4 exf2+ 7.Ke2
fxg1=N+ $1 8.Ke1 Qh4+ 9.Kd2 (9.g3 Qe4+) Nc6 $19


Oddly enough, I happened across that line for the first time just earlier today.

Oh, and there are situations where underpromotion to a bishop or rook is necessary in order to avoid a stalemate that a queen might create. For instance, in the sequence of moves coming from the Saavedra position, if white were to create a queen with 6. c8=Q, this would allow black to draw by stalemate, and so white must instead play 6. c8=R in order to win:

[FEN "8/8/1KP5/3r4/8/8/8/k7 w - - 0 1"]

1.c7 Rd6+ 2.Kb5 Rd5+ 3.Kb4 Rd4+ 4.Kb3 Rd3+ 5.Kc2 $1 Rd4 $1
6.c8=R $1 (6.c8=Q $2 Rc4+ $1 7.Qxc4 $10) 6...Ra4 7.Kb3 $18