Sabermetrics

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

SportMetrika, Inc. - The corporate website for a team of consultants to sports organizations.
http://www.sportmetrika.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/

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/

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/

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/

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

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

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

Original Baseball Research - Personal site of Clifford Blau, who has made his sabermetric studies on such topics as Bill Mazeroski's fielding, the effects of artificial turf, and park factors available.
http://mysite.verizon.net/brak2.0/

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/

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/