FanGraph (FG)
http://www.fangraphs.com

BaseballReference (BR)
http://www.baseball-reference.com/

The Hardball Times(THT)
http://www.hardballtimes.com/main/stats/

Retrosheet
http://www.retrosheet.org/

http://www.statcorner.com/

http://www.highheatstats.com/

http://www.hittrackeronline.com/

http://minorleaguecentral.com/