[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, precompiled debs are available for x86, amd64 and ppc at http://www.debian-multimedia.org/
There is a MPlayer plug-in for ["Mozilla"].
See also: MPlayer
Mplayer in Debian
See the [http://people.debian.org/~mjr/mplayer.html Debian mplayer FAQ] for some background information about the Mplayer in Debian saga.
The proposed source package has been stripped from at least encrypted DVD playback and mencoder.
In September 2006, there was an [http://lists.debian.org/debian-devel/2006/09/msg00915.html update and summary on the case] on debian-devel.
The package landed in Debian Sid on October 26th 2006, this was [http://bugs.debian.org/113238 one of the longest ITP] (a little more than five years).