I suprised myself and won one game in Azoth's Resourceful Master Tournament, but I was wondering why my opponent resigned. He has an advanced pawn and one of each of his bishops and rooks is posed to attack my king's position. I am a knight up, though.

 

You can force exchanges down to a won endgame.  your king is safe.  You have an extra q-side pawn.  And youre a piece up.  he is right to resign.