Getty Images: Getty Images has recently expanded their open access policy on the bulk of its collection for personal or non-commercial use (only) with attribution. When conducting your search the media type is automatically set for Creative Images, Rights-Managed and Royalty Free.
LIFE Photo Archive (hosted by Google): includes millions of photographs many of which were never published by LIFE magazine. Its archive stretches back from the 1750’s to today.
Everystockphoto: a search engine for free photos from many sources. You can search by license type in the advanced search feature.
Calisphere:digital collections from California's libraries, archives, and museums, with over 400,000 images, texts, and recordings.
Unsplash: free high-resolution photos, adding 10 new photos every 10 days, licensed under Creative Commons Zero.