Sabermetrics

The Baseball Think Factory
The baseball braintrust gets together to create this site. Player comparisons, and statistical information kept current. Articles from the baseball minds are thorough and informative.
http://www.baseballthinkfactory.org/btf/

The Baseball Analysts
Rich Lederer and Bryan Smith contribute daily to this part-weblog, part-science project, keeping you up to date with the days statistics of interest.
http://baseballanalysts.com/

Baseball Graphs
Gain clearer intuition about baseball by using innovative ways to visualize baseball statistics.
http://www.baseballgraphs.com/main/index.php/site/

The Hardball Times
This baseball digest gives daily doses of analytical observations from the game of baseball.
http://www.hardballtimes.com/

High Boskage House Baseball Analysis Web Site
Statistics are updated daily. Comparisons by position, league, and historical data. Articles on theory and physics can also be found here.
http://highboskage.com/

Red Sox Stats
Calculations for the Red Sox organization. Also statistics for all MLB players.
http://www.redsoxstats.com/

RIOT -- Major League Baseball Playoff Races
UC-Berkeley's Industrial Engineering and Operations Research department has created their own daily calculation metric for analyzing team record performance based on historical data. Results include the number of wins needed to date to guarantee a first-place division finish or playoff run, as well as to avoid elimination from first or the playoffs. Calculations provided thanks to the research of Professor Dorit S. Hochbaum.
http://riot.ieor.berkeley.edu/~baseball/

SABR/Bob Davids Chapter
Regional SABR chapter covering major and minor league baseball teams in the Mid-Atlantic area.
http://www.sabrdc.org/

Society for American Baseball Research (SABR)
Research information, SABR on-line bookstore, on-line membership application, bulletin board comments page, and baseball links.
http://www.sabr.org/

Society for American Baseball Research (SABR) UK
An archive of research articles and illustrations and information on how to join the UK chapter of SABR.
http://www.sabruk.org/

Tango On Baseball
A collection of various research about linear weights, runs, and wins.
http://www.tangotiger.net/