Philip Humber pitched the 21st perfect game in baseball history, and the fourth since Mark Buehrle threw one for the White Sox in July, 2009. Here's a list of perfect games in baseball history, with help from Baseball Almanac:

Lee Richmond - 6/12/1880
John Ward - 6/17/1880
Cy Young -5/05/1904
Addie Joss - 10/02/1908
Charlie Robertson - 4/30/1922
Don Larsen - 10/08/1956
Jim Bunning - 06/21/1964
Sandy Koufax - 9/09/1965
Catfish Hunter - 5/08/1968
Len Barker - 5/15/1981
Mike Witt - 9/30/1984
Tom Browning - 9/16/1988
Dennis Martinez - 7/28/1991
Kenny Rogers - 7/28/1994
David Wells - 5/17/1998
David Cone - 7/18/1999
Randy Johnson - 5/18/2004
Mark Buehrle -7/23/2009
Dallas Braden - 5/09/2010
Roy Halladay - 5/29/2010
Philip Humber - 4/21/2012

Humber's perfect game was the third in White Sox history, the most by any franchise.

Not on this list, of course, is Armando Galarraga, who lost his perfect game on a blown call at first base. Two pitchers -- Harvey Haddix and Pedro Martinez -- were perfect through nine innings (with Haddix perfect through 12) before losing the perfect game and the game in extra innings.