Baseball Almanac (website)
Baseball Almanac is an in-depth website with the tagline "Where what happened yesterday is being preserved today." It is known for its player pages, numerous lists and sundry other features. It covers myriad aspects of the game, from college ball to the major leagues.
It is located at http://www.baseball-almanac.com.
It also hosts Baseball Fever, a popular baseball discussion forum that features numerous well-known members.