MPlayer was the best video player for a long time. Then a fork happened and many developers left for MPlayer2. That was in turn abandoned for mpv which is now the most updated and feature-rich branch. MPlayer is still updated and alive but updates are essentially just small fixes to make sure it still compiles against modern versions of ffmpeg.
MPlayer comes with a video conversion tool called mencoder. This has also not been more than barely maintained for decades.
The modern alternatives
- mpv is a better and more updated stand-alone player than mplayer (and mplayer2)
- SMPlayer is a better choice than GMPlayer if you want a video player with a graphical interface
- ffmpeg is a far better tool for converting audio or video than mencoder.
- mplayer's website: http://www.mplayerhq.hu/
- mplayer and mencoder manual