[http://mplayerhq.hu MPlayer] is a powerful Multimedia player and more. - It plays almost anything from RealPlayer to avi, but has licensing problems which makes it currently unsuited for going into Debian/main.
Also it has an Encoder shipped with it ["MEncoder"], which is very powerful.
Building your own package by yourself for Debian is easy, though, and if you're really lazy precompiled debs are available for x86 and ppc at [http://debian.video.free.fr/ http://debian.video.free.fr/]
There is a ["MPlayer"] plug-in for ["Mozilla"].
See also: MPlayer