KDE has an open source video player named KMPlayer (not to be confused with Pandora KMPlayer). KMPlayer is actually a GUI for MPlayer. So, installing KMPLayer will also install MPlayer and many KDE dependencies. KMPlayer is available in Ubuntu official repository. Here how to install it.
Install Via Terminal
sudo apt-get install kmplayer
This will download approximately 86 MB of packages and use 230 MB space after install.
Install via GUI
Open Ubuntu Software Center and search for KMPlayer then press install button.
You can right-click any video file then choose open with KMPlayer. Or you can open KMPlayer first from menu.