Microsoft's Official Xbox site
Xbox.com is Microsoft's official website dedicated to the Xbox. There is a ton of information here along with forums and news about upcoming Xbox games.
360 Voice is a website where you register your Xbox Live Gamertag, and your Xbox 360 automatically blogs about what you played and how many points you earned. Very cool. You can also setup Gamerscore challenges here as well.
CheapAssGamer should be your first stop if you are looking for cheap games. The forum community is great as well.
The Sim Exchange
The Sim Exchange is a stock market trading game where you buy and sell shares of videogames rather than companies. It is very fun and a neat game that causes you to look at the industry in slightly different ways.
Game FAQs is where you will find hints, tips, cheats, and walkthroughs for pretty much every game out there.
Rooster Teeth is where you'll find the kings of machinima, the masters of videogame movie making, and the arch dukes of silly videos. Best known for making the Red vs. Blue series, Rooster Teeth also has a regularly updated web comic among many other projects.