Google Vision AI - Cloud Vision API provides a comprehensive set of capabilities including object detection, ocr, explicit content, face, logo, and landmark detection.
AllTheFreeStock - All the free stock images, videos and icons in one location
CloudSight - Image recognition API; send an HTTP request with an image, get a description of contents.
CaptionBot by Microsoft - Analyzing and describing images by a bot
Amazon Rekognition - Add Amazon's advanced image analysis to your applications.
Imagga - Advanced image recognition technology wrapped in powerful API.