Recommended Links:

mtgontario.com
Magic The Gathering Ontario is a local site dedicated to the Magic scene in Ontario. A great resource for tournament listings in the area which are always followed up by comments on the forums as well as a tournament report or two. My regular Multi-player Miscellany column focuses on casual Magic play and appears every other Monday.

starcitygames.com
A very popular site with tons of format strategies from casual to pro, Pro Tour coverage, and great discussion threads taking on very relevant issues.

brainburst.com
Another popular site with lots of deck lists, many message posts on building those decks, and strategy articles from Pro players.

themanadrain.com
The top site for Type I players. Listings for all the major Type I tournaments in North America. Membership is needed to post on the forums and getting a member is hard as they are very selective with who can join.

magicthegathering.com
The official Magic: the Gathering™ site with daily articles from the game developers themselves.

thedci.com
All the official tournament rules for every sanctioned Magic format as well as The Oracle card text reference, judge certification program, player DCI ratings and lots more.