Compositing nodes to do greenscreen (and bluescreen, redscreen etc) chroma-keying within material nodes, meaning you can place greenscreen video in your 3D scene without having to render out a png-sequence beforehand.  Works in Eevee and Cycles.

This allows you to create smooth CGI camera moves while filming your actors with a still camera, and have your live-action actors cast shadows, receive shadows and have reflections within your 3D scene.