The Swiss-Army-knife of video/audio processing. Almost every piece of media on the internet has been through FFmpeg.
From Wikipedia
FFmpeg is a free and open-source software project consisting of a suite of libraries and programs for handling video, audio, and other multimedia files and streams. At its core is the command-line ffmpeg tool itself, designed for processing video and audio files. It is widely used for format transcoding, basic editing, video scaling, video post-production effects, and standards compliance.