aptitude install firefox-3.5 (Ubuntu 9.04 shipped with Firefox 3.0 by default).
Nightly builds of Chromium: https://launchpad.net/~chromium-daily/+archive/ppa
miro (simple "internet TV" video podcast player)
thunderbird, gnumeric, abiword (my preferred lighter-weight alternatives to the default Evolution and OpenOffice apps)
gnome-do (like the OS X app Quicksilver)
inkscape (vector drawing)
git, git-svn, mercurial, darcs, bzr, subversion, cvs
irb, rubygems
This software covers most of my office/design type needs.
I use Google Chrome for normal browsing and Firefox for development, so I would try to get Chromium working.
http://code.google.com/p/chromium/wiki/LinuxBuildInstruction...
I also use vlc for playing DVDs, video, and audio clips.
http://www.videolan.org/vlc/download-ubuntu.html
There aren't really any killer opensource apps that Ubuntu is missing. The fast majority of what you will need for development is available in the repositories.
Chromium is also a must.
Except "Ubuntu-must-have-apps" is just as bad! The hyphen goes between "must" and "have" to signal that "must" is part of an adjective and not a verb!