Gatherer, Wizard's run card database including original vs rule-edited text, alternate artwork, and information about format legalities for almost every card printed to-date (except some promos I guess?): http://gatherer.wizards.com/Pages/Default.aspx
TCG Player- Card Seller Aggregation/sales with pricing history data/collection tracker: http://www.tcgplayer.com/
Other sites for businesses centered around Magic's secondary market- both sponsor players to compete on the professional level and both will sell supplies (cards, mats, sleeves, etc.): http://www.starcitygames.com/index.php
Rules questions answered by a magic judge (similar to blogatog but more rules-y and with a finnish lady instead of Maro): http://magicjudge.tumblr.com/