NFL Win Probability Calculator

Two things I've been interested in for a long time are NFL football and mathematics. This makes a site like Advanced NFL Stats right up my alley. In particular I like the Win Probability Calculator that was recently posted there. You enter a game state, including the score, field position, and time remaining, and the system produces the probability of each team winning the game. I think this could be a great tool for arguments about going for it on 4th down, or for attempting 2-point conversions.