This is one of my favorite multimedia player for Linux ever. It is actually a neatly built front end of Mplayer, a popular linux multimedia application. I strongly suggest newbie ubuntu users to get a taste of this super cool mutiledia application.
Install the latest version of smplayer in Ubuntu.
VLC Multimedia Player
I guess, those who haven't heard about VLC multimedia player is probably a total newbie to the computer world as well. VLC supports almost all kinds of media formats and a complete must have.
And for those who are wondering about the movie shown in the above screenshot, it is called Big Buck Bunny, a movie made completely using open source tools. More on movie here.
Install Brand New VLC Player in Ubuntu.
Rhythmbox comes as the default music management tool for Ubuntu. IMHO I think there is a number of other really good music management tools in Ubuntu. Banshee is one among them. Banshee is rather simple and easy-to-install-and-use kind of media player, which everyone will love. There is already talks for including Banshee as default music management tool for Ubuntu from next release.
Install latest version of Banshee in Ubuntu.
Amarok is another good alternative to Rhythmbox. I used to be a huge fanboy of Amarok 1.0 series, but Amarok developers have decided upon a complete overhaul of this awesome music management tool starting from Amarok 2.0. That is a welcome development. But the problem is, even though Amarok 2.0 looks much better than the Amarok 1.0, it has in turn become highly unstable and buggy. So to conclude, although Amarok have lost a lot of its charm, it is still a good alternative. EDIT: Try Clementine also.
Install Latest Amarok in Ubuntu.
Kdenlive Video Editor
Video Editing Apps in Linux are not yet the best you could get. Openshot 1.2 has been released recently, which is a pretty decent video editor for Linux. But it still has got to go a long way. As of now Kdenlive is the best Video Editor app you could get in Linux.