Sabermetrics comes from the acronym for the Society for American Baseball Research, SABR. Sabermetrics is the statistical study of baseball; techniques of probability and statistics are used to discern real trends in baseball [for example, the qualities of hitters] from those that just seem to happen [for example, the quality of a player to "hit in the clutch"]. As such, sabermetrics is an oft-debated field of study.
Baseball Prospectus Online
In-depth columns, prospect interviews, transaction analysis, and player cards containing Baseball Prospectus's Davenport Translations. Metrics for evaluating hitters, starters, and relievers updated daily; four to ten columns presented free of charge on the site per week.
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.
The Hardball Times
This baseball digest gives daily doses of analytical observations from the game of baseball.
Owlcroft Baseball-Analysis Site
Comparisons by position, league, and historical data. Articles on theory and physics can also be found here.
Red Sox Stats
Calculations for the Red Sox organization. Also statistics for all MLB players.
Society for American Baseball Research (SABR)
Research information, SABR on-line bookstore, on-line membership application, bulletin board comments page, and baseball links.
Tango On Baseball
A collection of various research about linear weights, runs, and wins.