Baseball Almanac - An excellent resource for information about ball players, teams, stadiums, and other information related to the history of baseball.
Baseball Reference - Contains current and historical statistics for players, teams, and games.
Baseball Savant - Producer of Major Leage Baseball's statcast. In depth source of statistics and metrics related to MLB.
Historic Baseball - Dedicated to the history of the game. Learn about baseball through the centuries, as well as various leagues, teams, and players.
National Baseball Hall of Fame - Check out their digital collection and plan your visit to Cooperstown, NY.