FFmpeg is the leading open source multimedia processing library. Nearly any open source program you use that can play a sequence of video images accompanied by sound is probably using FFmpeg to do so. It can decode the most prominent multimedia formats ever created, whether open or proprietary, and can encode to the most important formats as well. As a novelty, FFmpeg can also decode a vast array of impossibly obscure multimedia formats and remains essential for legacy multimedia files (no codec is too old or obscure for this project). Anecdotally, FFmpeg just might be the leading multimedia processing library in existence, as many proprietary applications use it as well (as permitted by the LGPL license). There are many stealth uses as well. YouTube does not advertise how their backend conversion software operates. However, independent, empirical research has demonstrated that YouTube uses FFmpeg to convert videos.