[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, amd64 and ppc at [http://www.debian-multimedia.org/ 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 current proposed source package has been stripped from at least encrypted DVD playback and mencoder. A [http://tonelli.sns.it/pub/mplayer/etch/ mplayer Debian package] is available from the maintainer's website.
There is some recent (as of September 2006) [http://lists.debian.org/debian-devel/2006/09/msg00915.html update and summary on the case] on debian-devel.