The Met recently released a collection from it's archives for non commercial use. They've released 400,000 images total and 18,000 photographs. It's a great resource for students and artists looking for inspiration or images they can use for research. These were some of my favorites from the photography collection. I highly recommend browsing through them to see for yourself.
Another photo archive I've really enjoyed is National Geographic's Found. It's a great archive that spans over 100 years and I love their color images! I'd post some if copyright laws allowed but for now you'll have to check it out yourself.