The Bobby Knight 'critics can kiss my ass' speech was one of my favorites - glad someone here mentioned it.
From recent years, I have to nominate Jim Fassel's 'This team is going to the playoffs...' speech a few years. This was when the New York Giants were in a slump and coach Jim Fassel delivered this speech. After that, the Giants won every game and went to the Super Bowl.
Don't remember the whole thing exactly but some of it went like... "This team is going to the playoffs...If this is a poker game, I am putting all my chips in the middle of the table...If you want in, get in...If you want out, get out..."