From the About-page:

Photos for everyone

Over 3 million free high-resolution images brought to you by the world’s most generous community of photographers.

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Is it really free? Yes.

Unsplash is a platform powered by an amazing community that has gifted hundreds of thousands of their own photos to fuel creativity around the world. So sign up for free, or don’t. Either way, you’ve got access to over 3 million photos under the Unsplash license—which makes them free to do-whatever-you-want with.