It's been a quick transition from app provider to API provider for photo-sharing Instagram and its Instagram API. Today the company launched a new Instagram Real-time API, which provides access to the site's content via webhooks as photos are added that match pre-determined criteria.
Google Cloud Vision API is a REST API that will help developers analyse images like never before. It supports various features like Entity identification, Facial Detection, Landmark Detection, Logo Detection, OCR and Identifying Explicit content. It is available in Limited Preview for now.