Win Probability Added. It measures the increased likelihood of your team winning the game, based on your performance, so a three-run homer when down by two in the ninth is worth far more than the same homer in a 14-2 blowout. It's developing, and the further back we go, the more unavailable (or corrupted) play-by-pay data is, so latter-day players will dominate. They also don't do much of a job measuring defensive contributions, so these may even out as they find a manner to extract fielders' defensive performances from pitching and add it to the the offensive WPAs of batters. batmags, among others, doesn't like it (and please correct me if I'm wrong) because it measures clutch performance, which he contends is giving players credit for random variation, as clutch performers don't really exist (even if clutch performances do). I rather like it, because if clutch performance is merely random variation, it should be corrected for in a large data set, and if it isn't, well, here we have a measurement for it. I further like it because whether or not you hold that some guys are awesome because of their particular ability in the pinch, we have to find a way better than WAR to weight the performances of guys whose entire job is designed to perform in high leverage situations --- relief aces and pinch-hitters. So I appreciate seeing Tug McGraw appearing twice on the above list.