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. 

http://www.members.cox.net/boxer561/

--Tony Triem is a boxing historian who has a nice site dedicated to the memories of some of the great but not household names in boxing.   He also has an archive of materials if you are looking for data on a specific boxer.