Fastball Reaction Time - Use this game to test your reaction time and see if you can hit the ball out of the park. The game will show you your reaction time to each "pitch." From San Francisco's Exploratorium.
Graphical History of Baseball - Grades 6-12 - The design is really basic, but this site offers on-line graphs comparing baseball data like batting averages over the past century. It would be a great starting point for a lesson in interpreting graphs.
Math League Baseball - Grade 8 - Here's a five week, standards-correlated unit for 8th graders on the mathematics of baseball. Teachers could either use the entire unit or excerpt elements for shorter lesson units. There's a whole field full of good ideas in this one!
Math Baseball - This is an on-line game in which students advance runners and score runs by completing math problems of varying difficulties. Users can select easy or difficult problems, either non-algebraic or algebraic, and watch the runners move as the scoreboard changes.
Batter's Up Baseball - Grades 4-8 - Here's another Flash-enhanced math game built on a baseball theme. The problems (addition, subtraction, multiplication, division) presented vary widely in difficulty, so the game could be used across several grades or with students of varying ability.
San Francisco Baseball & Math - Grades 4-8 - This site asks users to solve math problems by scouring the San Francisco Giants' web site for prices, statistics, and other information. A good baseball web quest and math lesson.
Baseball Statistics Menu - This section from the Baseball Almanac offers definitions of some of the more common - and also more obscure - offensive, defensive, and pitching statistics. Lots of possibilities here: let students create formulas from the definition, compare stats for their favorite teams, etc., etc.
Baseball Reference - Lots of information and statistics in this site. Math teachers, have at it!