Unsplash has its own license, which essentially lets you use the images for free.

Their vast, well-organized collection of free stock photos is one of the best curated on the internet.

Pixabay hosts more than 2.6 million photos, vector graphics, and videos—all of which are free to use.

PicJumbo offers a variety of free images for any kind of use - free of charge with no registration required.

Over 25 million Free Graphic Resources for download. Find illustrations, vectors, stock photos, PSD, and icons.

Stocksnap uses the Creative Commons CC0 license so its photos are free to download, edit, and use.