Here are some sites you might find interesting. Some are sites maintained by other collectors whom I've either bought or traded cards with or whose sites I've come across for some reason.

www.NowBatting19.com - Tim Pulcifer maintains an interesting web site that provides interesting information about the history of baseball cards and other related topics. I came across his site while searching for information about some error cards I have. We had some interesting conversations and agreed to exchange links.

www.KenmoreCollectibles.com - Peter and Mike run this sports card store practically in the shadow of Fenway Park. Kenmore Collectibles was formerly JJ Teaparty, on Bromfield Street in downtown Boston. I find them to be honest card sellers. When you're in the neighborhood, stop in. They're at 466 Commonwealth Avenue, about a block or two east of the ballpark.