A metric that determines the value of a player by (theoretically) putting him on the basketball floor with four "average" players, and compares that to the team results if those four average players were on the floor with a "replacement-level" player. A replacement-level player is one considered to be below average.
WARP is another metric that has borrowed from previous work done in the sport of baseball. The website Baseball Prospectus defines replacement level as, "The expected level of performance a team can receive from one or more of the best available players who substitute for a suddenly unavailable starting player at the same position and who can be (or were) obtained with minimal expenditure of team resources."
Through mathematical manipulation, the "value" of a replacement player can be determined for basketball. Kevin Pelton, who is credited with the creation of WARP, has estimated replacement-level winning percentage to be 41.5.
The advantages of this metric are that it can be used to determine a player's contribution in a number of ways...offensively, defensively, on a per-minute basis, and overall.