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.