MPlayer is a free open source media player written in C and first released in 2000 primarily for Linux, though builds are occasionally made for Macintosh, Windows, and a variety of other platforms. The player supports a wide variety of audio and video codecs with many different options. Unfortunately, development on the project has slowed to a crawl and it hasn't been updated since 2019.
In the 2000s, a co-worker of mine told me about MPlayer as a one-stop replacement to the variety of media players I had been using up until that time. After I installed it, and found it could play every video file I had on my computer, I was able to overlook the poor interface and made it my sole video player. However, over the next couple years I began to grow tired of the fact that the project didn't produce their own binaries, and their suggested third party build site was always behind in updates. It didn't take me long to find a replacement with VLC Media Player which rectifies those problems. For awhile I kept MPlayer installed on my computer and used it for the very few formats VLC couldn't play properly, but, VLC eventually got around to supporting those too, and, when I got my next PC, I never bothered to install MPlayer, and haven't since.