Flickr is an amazing and huge online photo-blog system. While all images on this site are the properties of their respective artist, and should be respected, this site is an invaluable ‘research’ tool for finding pretty much photographs and images of almost any topic.
A long-running, uber-photo blog service. A running count on the home page gives you the latest stats of the site - When I last visited there were over 171 Million photos on the system. Another great research site when looking for reference material
Morguefile offers royalty-free stock photographs for download, donated by online photographers around the world! If ever in need of a background plate or material for creating matte paintings with, images for a texture, or even just for research purposes then this site is an invaluable resource.