Sean Lahman's Baseball Archive Excellent general baseball site. If you have a database program, he has a free downloadable baseball database with just about all the information you could want on anyone in baseball history. Lots of other stuff here as well. Great site.
baseball-reference.com The best site on the web for encyclopedic baseball information.
Strat Baseball Mailing List Searchable Archive Not sure how useful this will be to people. Certainly my own notes are better and I can probably answer or at least find the answer to any Strat-specific question you might have. But it is here and I've found at least some interesting opinions on the occasions I've searched it.
Strat Defense Article "When To Play the Slick Fielder Over the Heavy Hitter." Excellent article on the balancing of defense and offense in Strat. The short answer is that offense is usually more important, but of course positional differences apply. Unfortunately, I don't believe the offensive information needed to fully utilize this system is available on any of Strat's pregenerated reports. I can, however, probably tally up the info you need upon request if you wish to use this system. Remember, though, when requesting this information that I only have a limited amount of hours in the day. Please don't ask for the calculations required unless you're on a genuine guess. (I.e. if a simple glance at the stats shows that one guy has a huge offensive edge and the other has a small defensive edge, you know who you're going to play without bothering to run the numbers.)
Gary's Strat-o-matic Baseball Web Page Some useful information here. More for me as Commissioner than for league members, but still. This is a good place to look to compare Strat's park ratings as well, since he has a list of them for every park every year. There's other useful info here as well. Good site.
Strat-o-matic Home Page Very mediocre site. I only use it to read an occasional article (of dubious quality usually) and to order my disks from. I include it here solely for the sake of completeness.