Google together with Life magazine has published a photo archive of over more then 2 million photos. They promise that this collection will expand to 8 million.
Check it out via this link: http://images.google.com/hosted/life.
In this collection you will find photos that were published in Life magazine and also many unknown, never published before photos. Many of my favourite photographers can be found in this collection. It is a delight to scan thru the immense database.