MKV can be played by both mplayer and I believe VLC now as well(They had a bug preventing it for a while but I think that got fixed an update or two ago).
MKV for those unaware is the MatrosKa Video container, and is IMO one of the better container(My preferred Video container) as it can obviously store a variety of compressions such as h.264, but also handles seperate streams, chapter markers, etc all very nicely. And is open to boot

Seablade
Who is already partway through re-encoding all his videos into h.264 mkv files on his server.