used to be the best(IMO) but they just redesigned their website and it sucks, but they have live reports every day, so that is cool to read as the race unfolds.
www.velonews.com-has live reports as well
www.pezcyclingnews.com-probably the most enjoyable to read because the writer's have a sense of humor, and the 'daily distraction' is always a plus.
There are a bunch more, letour.fr is awesome if you can read french, very in depth. The american version is pretty good too, but not as in depth.