Elite Fitness Systems: A great site with all the equipment you will ever need, great articles and a pretty exhaustive exercise index. Definitely worth checking out.
TNation: This is an online magazine that comes out every Monday. It has tons of articles on training, nutrition, mass gaining, pretty much everything you would want to read about in regards to diet and exercise. One of my favorite things about this website is that there is not just a single school of thought. There are articles from power lifters, body builders, strength coaches, etc. You get to access the minds of some of the worlds best coaches. I frequent this site pretty regularly.
Wannabebig.com: Lots of videos and training programs.
Fitcast.com: Great podcasts!
John Berardi: The man when it comes to nutrition