Mplayer is a  popular movie player for Linux that supports reading from network and a wide range of codecs.

Install Mplayer:

First,execute the following code and make sure the two lines below the code exists in the next window:

sudo gedit /etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu jaunty universe multiverse
deb-src http://archive.ubuntu.com/ubuntu jaunty universe multiverse

Then,run the command:

sudo apt-get update

and install Mplayer by the code:

sudo apt-get install mplayer

After installed, you will find it at Applications–>Sound&Video–>MPlayer Movie Player

Install libdvdcss2 and w32 video codecs:

You need to install w32codecs for playing WMV,RealMedia and other formats files and libdvdcss2 for playing encrypted DVDs.

Execute following command to add the GPG key for ubuntu 9.04 users:

sudo wget http://www.medibuntu.org/sources.list.d/jaunty.list --output-document=/etc/apt/sources.list.d/medibuntu.list

sudo apt-get update

sudo apt-get install medibuntu-keyring

sudo apt-get update

And install codecs by command,

for i386 users:

sudo apt-get install w32codecs libdvdcss2

for amd64 users:

sudo apt-get install w64codecs libdvdcss2

Now,open a file folder and right click on a movie file to go to Properties.Tick on MPlayer Movie Player at Open With tab, and enjoy yourself by clicking on the movie file.

Tips:

You may want to install mlpayer plugin for Firefox,execute following command in terminal:

sudo apt-get install mozilla-mplayer

To make mplayer looking good by installing mplayer skins