In 1900, the National League contracted to 8 teams, and in 1901 the American League joined as the "Junior Circuit" with 8 teams as well. The league remained at 16 teams until 1961.
Still very much in the dead-ball era, the 1900s was a decade dominated by pitching, with two of the best of all time in Christy Mathewson and Cy Young both putting up phenomenal numbers.
That said, the player of the decade has to be "The Flying Dutchman," Honus Wagner. The consensus choice for greatest shortstop of all time, Wagner won seven batting titles in the decade and added 487 steals to his numbers.
The star power of this era was somewhat lacking, so there were not too many notable omissions, but the Cubs' famed trio of Joe Tinker, Johnny Evers and Frank Chance are worth mention as well as Hall of Fame right-hander Jack Chesbro.