Unsplash is an open collection of photos and illustrations contributed by photographers. Its developer page counts 9 million photos from over 432,000 photographers and almost 30,000 applications that use its API (an interface that another program calls over the internet).
The Unsplash API is a JSON API at api.unsplash.com; Unsplash runs its own website on it. An application can search photos by keyword, get a random photo filtered by search term, orientation or collection, list new photos and manage collections. Search uses tens of millions of tags from the community and image recognition. Official libraries exist for JavaScript, PHP, Ruby, iOS and Android. The application must embed the image addresses returned by the API directly (hotlinking), so that Unsplash can count views for the photographer, and it must credit the photographer and Unsplash. A new application starts in demo mode, limited to 50 requests per hour; after approval for production the limit is 1,000 requests per hour.
The images are free to use for most commercial and personal projects. Registering as a developer and using the API costs nothing; Unsplash+ is a separate paid subscription.