With such a wonderful Open Source community, wonderful programs for Ubuntu are appearing left and right. Here is my list of the best add-on installations for various categories:

Games:
-Tremulous: A wonderful 3D egoshooter, play as an alien or a human
-Glest: Warcraft on steriods, awesome graphics, great gameplay
-Wormux: a fun Linux Worm-clone

Media:
-Cinelerra: a very promising non-linear video editing tool that looks, feels, and works like FinalCut
-Avidemux: a nice, linear video editing program similar to MS Video Editor
-VLC: a media player that comes with its own codecs and plays practically anything you can throw at it.

Graphics:
-CompizFusion: comes standard with Ubuntu Gutsy, most people don’t unleash it true potential due to lack of configuration. A great GUI can be installed by running: sudo aptitude install compizconfig-settings-manager

Education:
-Stellarium: An awesome 3D planetarium

Internet:
-Google Earth: Lets you fly from New York to Helsinki in less than10 seconds

Others:
-Wine: Runs many Windows programs inside Ubuntu, keep up to date and use their official repository at: http://wine.budgetdedicated.com/apt
-Ubuntu Tweak: Lets you tweak various aspects of your OS
-InnoTek VirtualBox: Allows you to run Windows and other OSes inside Ubuntu.