Flickr.com from Yahoo is a popular photo sharing site which hosts over 5 billion photos. Visitors can create a free account with Flickr to upload photos, search and discover photos and share their photos.
Explore photos based on places, last 7 days, this month, popular tags etc and it has over 10 million active groups and learn from 60 million photographers. It also has sections like find friends, calendar, video on flickr, getty images, organize photos, favorites, stats etc
Web : http://www.flickr.com/