A listing of data resources and online sports statistics forums for a number of sports, available online. 

General Sports Sites

• Hickok sports
• Massey Data (see "tree" menu at the bottom of the page)
• Sports-statistics
• Github Repo (maintained by Ben Baumer)

Sports Statistics Blogs and Online Forums

• Harvard Sports Analysis Collective

Baseball Sites

• Retrosheet
• Baseball-Reference.com
• Baseball Statistics from CNN/SI
• John Skillton's Baseball Links
• MLB History
• SABR: The Society for American Baseball Research
• Sean Lahman's Baseball Archive
• Total Baseball
• Doug Steele's MLB and NBA statistics

Basketball Sites

• Current NBA Stats
• Final Four
• NCAA Final Four
• Basketball-Reference
• 82games.com
• Doug Steele's MLB and NBA statistics
• Ken Pomeroy's college basketball site
• basketballvalue.com
• APBR

Cycling Sites

• Union Cycliste Internationale

Fencing Sites

• United States Fencing Association

Fighting Sites

• UFC Fight Statistics

Football Sites

• NFL Statistics
• Pro-football Reference
• College Football Data Warehouse

Golf Sites

• Golf.com PGA Tour Statistics
• GolfWeb
• Shotlink Data (site provides instructions for obtaining data)

Hockey Sites

• NHL Players' Association Statistics
• QuantHockey.com
• hockeyDB.com

Horse Racing Sites

Olympics Sites

• Official Olympics Site

Soccer Sites

• Albyn Jones' Soccer Links
• Soccerway
• World Cup Soccer

Tennis Sites

• ATP World Tour results archive
• WTA tournament archive

Volleyball Sites

• Volleyball Worldwide