OpenCV - OpenCV is the world's biggest computer vision library
Microsoft Computer Vision API - Extract rich information from images and analyze content with Computer Vision, an Azure Cognitive Service.
Amazon Rekognition - Add Amazon's advanced image analysis to your applications.
Clarifai - The World's AI
Google Vision AI - Cloud Vision API provides a comprehensive set of capabilities including object detection, ocr, explicit content, face, logo, and landmark detection.
SimpleCV - SimpleCV is an open source framework for building computer vision applications.