Thanks to Dave Studeman, there's now a calculator that gives you a players win probability above a bench player. Pretty handy-dandy. Here's his explanation:
The only other win stat I know that exactly matches each team's wins and losses is Win Probability Added (WPA). WPA is the difference between a player's Win Advances and Loss Advances in each game. By subtracting one from the other, WPA compares a player's contribution to an average (.500) player.
Well, what if you want to compare a player to another level, such as a .350 player, or .400? It's a lot more useful to compare someone to a lower level than average, particularly when players have played different numbers of games. Unfortunately, the math for replacement-level WPA isn't exactly straightforward, because teams compile win advancements even when they lose games. The typical team has 1.65 win advancements in a win and 1.15 win advancements in a loss.
Never fear. I've decided to teach myself PHP and MySQL programming this summer, so I've created the following little PHP script to calculate a player's WPAB. Win and Loss Advances are available from Fangraphs and you can select your own replacement level (please use the decimal point—.350 or .400).
The calculator can be found here. Monkeying around with it, here are the top ten hitters by this measure over the last three years:
In case you are wondering, I set replacement level to .380. Albert Pujols, the mang, the mangchine. A team of Alberts would go 210-0 every season.
Now for the laggards. Here are the worst five: