FFmpeg
Swiss army knife for video/audio processing. Fabrice Bellard started in 2000. Convert, cut, encode, decode 1000+ formats. The heart of nearly every video service.
CLI tool + libraries (libavcodec, libavformat). Used by: YouTube (transcoding), VLC, Plex, OBS, every streamer. Notoriously fragmented option flags. The libav fork existed 2011–2017 (Debian + Ubuntu temporarily switched). Bellard also wrote QEMU, TCC, JSLinux. Modern activity: active release cycle, AV1, H.266/VVC, modern hardware acceleration via VAAPI/NVENC.