The Body Is Not An Apology is a pretty serious website. Not only is it a website about "Radical Self-Love" for all kinds of bodies, but its creators believe that we can change the world by practicing acceptance. That's right. We can change the world just by being our awesome selves and encouraging everyone else to be their awesome selves.
On a far less serious, but still awesome note, Rookie is a really great online magazine. Their articles are about real things and regular people and the models don't all look the same. Fair warning though, you may find it addictive.
About Face is an absolutely fantastic website that looks at how women (particularly teens and young women) are portrayed in the media and advertising.
The You Are Beautiful project is pretty cool. I found out about it because one of my friends sent me some of the stickers. I promptly put one on my phone case so that my friends see it when I take a photograph of them.
It's not strictly about body image or body positivity, but the Scarleteen website has TONS of useful information on it, and is totally worth checking out. For example, this article about breasts is pretty fantastic.