Extrala: Probably the most active V:tES blogger around right now. Ralf nicely summarizes "important" tournament finishes, does a great job with "Deck of the Month" selections and nicely describes deck archetypes. My favorite V:tES web site these days.
Rulebook: The online version of the rulebook. It's lengthy, but it remains a good place to begin if you're either new to the game or looking for clarification on game mechanics.
Rules Reference: The Cliff Notes version of the Rulebook, an outline format of the way a game plays out.
Rules Playmat: A printable play surface which summarizes the basic rules of the game. Great as a teaching aid.
Individual Card Rulings: A card-by-card synopysis of formal rulings on cards, this should be mostly free of contradictions (unlike attempting to search newsgroups, where reversals sometimes occured and misinformation was common.)
Tournament Rules: Rules for the current "season," including banned cards and all the information necessary to play in a tournament
Judges' Guide: Basically, this is a continuation of the Tournament Rules. This adds section 10, which is required only for judges.
Usenet Newsgroup: If you can't find what you're looking for in the rules of the game or individual card rulings, there is a possibility it was discussed here in the past. Go to this site and search for keyword keyword author:LSJ in the newgroup. WARNING: Posts on the newsgroup were often part of a discussion that led to a ruling, and those rulings sometimes evolved in the course of the discussing. Be careful, what you find here may not be the current gospel.
Deck Building Utilities
Card Lists: Several formats are available, most commonly used to update more advanced deck building utilities.
Anarch Revolt Deck Builder: My deck building utility of choice. I find it to be very versatile, allowing serial and parallel searching. Includes a simple shuffle-draw utility, but no statistics.
Secret Library: An online deck building utility. Great if you like to share your decks with the community or provide links to decks that might be ungoing revision (it'll always link to thte current version.)
Ira's Probability Simulator: I don't personally use this, as I have calculate odds with HYPGEODIST in Excel. But Ira Fay's drawing simulator is convenient, easy to access, and fast.