Here is a primer for the uninitiated. We hope to add more fully to this as time goes on.
Glossaries
Baseball For Beginners -- A glossary from PBS for the absolute beginner
Baseball Glossary -- A site that presents a glossary of baseball's statistical elements
ESPN Classic Fantasy Baseball: Glossary -- A nice collection of statistical and situational terminology
Baseball Terms -- A rather nice collection of definitions explaining the language and jargon of baseball.
The Language of Baseball -- Dictionary and thesaurus with over 5,000 baseball terms
Rules
Major League Baseball : Official Rules -- The official rules from MLB.com
History
Baseball Almanac : The Official Baseball History Site
Ballparks
Ballparks by Munsey and Suppes -- From Bennett Park to Shibe Park, this site documents past, present and future MLB ballparks and stadiums
American League Ballparks -- Digital Ballparks.com site of current and historic stadiums
National League Ballparks -- Digital Ballparks.com site of current and historic stadiums
Players
The 400 Greatest by Total Baseball : A Legendary List on Baseball Almanac
Plays
Baseball's 25 Greatest Moments -- Dave Kindred's list from The Sporting News, Oct. 18, 1999.
Other Great Moments -- Clearly 25 wasn't enough, The Sporting News, Oct. 25, 1999.
Great Moments in Baseball -- From the BBC H2G2 Website, Jan. 26, 2004