Wall Street Sports - This site is my personal favorite. You buy and sell players on the open virtual market. Prizes are awarded for being in the top 100 performers.
Ebay - This is an awesome sight where you can buy and sell cards over the internet. It is a lot of fun to enter an auction for some great deals on collectibles and sports card.
Sports Illustrated for Kids - It has fun games to play over the net and information about players.