Win Shares is a book, published in 2001, and a Baseball statistic created by Bill James in the vein of Pete Palmer's Linear Weights method and the Value Approximation Method James previously advocated to assign a player's offensive, defensive, and pitching value one number.
A Win Share is, quite simply, one third of a win, as quantified by an incredibly complicated method that takes James pages upon pages to describe in full. In the book, which contains an encyclopedia of Win Shares prior to the 2002 season, a detailed breakdown of the Win Shares method, and several essays, James attempts to show all of the ways in which Win Shares can be used to analyze baseball. Trades can be "scored"; for example, take the 1997 trade that brought Mark McGwire to the St. Louis Cardinals in exchange for pitchers T.J. Mathews, Eric Ludwick, and Blake Stein. McGwire earned 10, 41, 30, 20, and 8 Win Shares before bowing out after the 2001 season due to injuries, a total of 109. Ludwick earned the A's 0 Win Shares before being traded to the Detroit Tigers for Kurt Abbott. Abbott earned them 2 Win Shares, and was then shipped to the Rockies for a minor leaguer who earned nada. T.J. Mathews earned them 3, 5, 7, 2, and 1 before being waived. That's 19 Win Shares so far. Stein didn't earn anything while with the Athletics, but was part of their pennant-drive deal to acquire Kevin Appier from the Royals. Appier earned 2 and 11 Win Shares before signing elsewhere as a free agent. The final score? 109-32. Not so great, although McGwire likely would have left for greener pastures following the season anyway. This is but one of the many applications he shows; other essays list the greatest fluke seasons of all time, find the average peak of a great player, and compare the Win Shares rookie of the year vs. the MLB rookie of the year vs. the best career debuting.
Win Shares have been met with mixed opinions, mostly positive; however, it's good to note that nearly every player in the top 50 is in the hall of fame or will be when their career ends, and the same can be said for many in the top 175.
Before I begin the randomness section of this writeup, here are some good guidelines for a typical season's worth of Win Shares. 20 is an all-star-worthy season; for a baseline, Scott Rolen, regarded by Win Shares as 2003's best 3rd Baseman, earned 25 Win Shares. 30 will get you close to an MVP Award in most years. Players who earned 30 Win Shares in 2003 include Gary Sheffield (35), Javy Lopes, (30), Jim Thome (30), and Alex Rodriguez and Carlos Delgado, who both earned 32. As I said, in most years this would warrant heavy MVP talk, and in the AL it will. However, the NL saw not one but two seasons that could be classified as "Legendary Seasons." Albert Pujols of the St. Louis Cardinals scored 41, the same as McGwire's 70 Home Run year. Barry Bonds, plagued by injuries and personal problems, still managed 39. This is rareified air; some teams have never had a player score this high in a season. The highest single-season Win Share totals are held primarily by 19th Century pitchers, who pitched nearly all of their teams' games and therefore receive all of the shares usually divvied between 10-12 pitchers now. In 1884 "Old Hoss" Radbourn set the all time record, going 59-12(!) and earning 89 win shares. But as far as today's standards go, the measuring stick is probably Honus Wagner, who hit and fielded his way to 59 in 1908.
Top Win Shares Player of All Time: Babe Ruth (608.75 batting, 44.72 fielding, 102.04 pitching, good for 756 when rounded into Win Shares form.
Highest Rated Player never to make it into the Hall of Fame: Pete Rose... I got this hot tip that he earned 547.
Most Win Shares by a Pittsburgh Pirate, single season: Honus Wagner, 59
(Here's the Cruel Part) Most Win Shares by a Tampa Bay Devil Ray, career: Fred McGriff, 66
Win Shares can currently be ordered only from the STATS, inc. web site. Win Share data from the 2002 season can be found at http://www.baseballtruth.com/bbt_winshares.htm# , and 2003 data can be found at http://www.baseballgraphs.com/winshares/ . Anything prior to that can be found in the book, which features every player ever dating from 1876 at the formation of the National League to 2001. Trade Data was found at the ever-awesome www.baseball-reference.com.