OpenCV
Open-source computer vision library. Gary Bradski, Intel, 2000. C++ core with Python/Java/JavaScript bindings. De-facto standard for image/video processing outside pure deep learning.
2500+ algorithms: edge detection, object tracking, optical flow, camera calibration, structure-from-motion, AR. Haar cascade classifiers (classic for face detection pre-DNN). The OpenCV DNN module (since 4.0, 2018) runs neural networks without TensorFlow/PyTorch dependency. Cross-platform: desktop, mobile (iOS/Android), embedded, GPU (CUDA). Competitors: Pillow (simple), scikit-image (Python-heavy), modern: Hugging Face Transformers (ML-specific), Roboflow (managed CV platform).