Equilibria in noncooperative games are typically inefficient, as
illustrated by the Prisoner's Dilemma.  In this paper, we quantify 
this inefficiency by comparing the payoffs of equilibria to the 
payoffs of a ``best possible'' outcome.  We study a nonatomic 
version of the congestion games defined by Rosenthal, and identify 
games in which equilibria are {\em approximately optimal} in the 
sense that no other outcome achieves a significantly larger total 
payoff to the players---games in which optimization by individuals 
approximately optimizes the social good, in spite of the lack of 
coordination between players.  Our results extend previous work on 
traffic routing games.