CloudSight - Image recognition API; send an HTTP request with an image, get a description of contents.
The Unsplash API - No-limits, do-what-you-want API for access to 200K+ HD pics.
Flico - Add visual search to your Chrome browser
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Google Vision AI - Cloud Vision API provides a comprehensive set of capabilities including object detection, ocr, explicit content, face, logo, and landmark detection.
API List - A collective list of APIs. Build something.