fineart
huge and rare collections of high resolution anatomy pictures [nudity]

suurland
blueprints of various cars, ships, planes and , military equipments

onnovanbraam
good collection of reference pictures from human body to cars etc

bonehub
strong collection of reference pictures, ideal for gameing artists

corbis
huge collections of photographs, a must see to get inspired

gettyimages
another well known photography site, millions of pictures from all fields

metagons
a good place for modeling reference including some wireframe too

sssbrno
very nice and detailed drawings of navy ships and turrets

greatbuildings
awesome collections of famous buildings, palaces of the world

dfrc
you will find some good quality NASA spacecraft and boosters

airshowreport
only aircrafts of any make

1000skies
only skies!, a must see to choose a sky for your scene

jeremy
good and rare collections of public photos

3dweave
good collection hdri photos, all free!

visibleearth
high resolution planetary pictures from NASA