Here are some links to sites that I find helpful, interesting or merely fun:

http://www.network54.com/Forum/579395/

--Run by Leon Luckey, Network 54's vintage card board has become a significant force in the baseball card arena and he's just launched a boxing card chat board.  Careful, though, as the knowledge is deep but the controversy can be just as deep there.

http://www.boxingcarddigest.com/

--My friend David Hanvey runs this site from Australia (G'Day, Mate!).  Lots of interesting features on international sets that I don't collect or cover.