The official site. From here you can reach:
The page for
Magic Online, if you want to give it a try. Note that, while you have to pay/trade for cards, there are bots who give them away for free. I don't have any experience with this, but there are people posting in this thread that do.
The DCI, for organized play.
Gatherer, WotC's card search.
magiccards.info, another place to waste lots of time browsing through cards. It doesn't have the user ratings and comments of Gatherer, but lists the prices of several online vendors and, surprisingly, has more card images. The interface is also better in my opinion.
MTGSalvation. That place has a lot of stuff, including a wiki, a huge forum, and many articles of varying quality. They also spoil all the cards of the next set well in advance, so this is where we'll usually get future cards from.
StarCityGames - they make you pay for much of their newer content, but what you can get for free is certainly good enough.
ChannelFireball.com, where you can hear LSV and Conley Woods (among others) discuss Magic. Many articles and draft videos from the pros are posted here for free. You can also buy cards from this website.
Elder Dragon Highlander, the official page. Always up to date and it has a forum about this popular variant multiplayer format as well. If you want to learn even more about the format, go here!
Le Bestiaire, an online draft simulator. It gives you some pretty odd ratings sometimes, but at least there is actual feedback.
Magic Workstation, a program for...a lot of things, including collection management and online play. Supports more TCGs than just Magic. There's a freeware version available.
Cockatrice, an other program for over-the-web Magic playing for no cost. Also has card images built in. Generally updates pretty frequently.
TC Decks, where you can see which decks have tournament success. The decks are essentially named by the people who play them, and if you're looking for, say, Legacy decks, you'll soon find out that not all tournaments are really at Pro Tour level, but this is still an invaluable site for anyone who wants to keep up with the tournament scene.
Magic: the Gathering Source Forums, which is great for people looking into legacy.
The Mana Drain, more forums, this time for people looking into Vintage.
Tapped Out, a deck building and critique community. Build any number of decks and put them up for review/critique/comment/display. Or, keep them private. They also have pretty graphic representations of your mana curve, colour costs and colour generation.
http://www.highlandermagic.info/ The site for German Rules highlander. It's a 100-card singleton format, but the rules are rather different from EDH. They're more in line with the normal rules, and the banlist is made with a more competitive mindset in mind.
http://deckstats.net/ Calculates mana curve, compares color spread to manabase colors, calculates prices for the deck as well as some other functions. It can handle MWSDeck files and can also save decks pasted into it in the format.