Viola-Jones face detection
The method detected faces in real time on an ordinary processor, discarding most of the image through a cascade of increasingly accurate checks.
Why it matters
Computer vision produced a result cheap enough to put in a device, and cameras started finding faces.
Three ideas together: the integral image, giving the sum of any rectangle in four lookups; feature selection by boosting; and a cascade whose early stages are cheap and reject almost everything. The method sat in compact cameras, phones and OpenCV for years. It stayed the standard until convolutional networks learned to find features themselves.