Magnum

An oldie but a goody, Magnum has been around since 1947, and their site showcases a good deal of the photographs created under the Magnum umbrella. Be sure to check out their Podcasts, and register to gain access to their archives of 350,000 images.
"Magnum is a community of thought, a shared human quality, a curiosity about what is going on in the world, a respect for what is going on and a desire to transcribe it visually." Henri Cartier-Bresson