Important: before running the commands below, you need to enable the restricted, multiverse and partner repositories. To do this, open the Ubuntu Software Center, select Edit > Software Sources, and on the first tab (Ubuntu Software), check all the repositories. Then switch to the second tab (Other Software) and check the "Canonical Partners" repository.
1. Install Adobe Flash, Java, fonts, codecs and other restricted stuff (I don't use "ubuntu-restricted-extras" because it also installs some stuff I really don't need while it doesn't install a lot of the applications that I do need):
sudo apt-get install gstreamer0.10-ffmpeg gstreamer0.10-pitfdll gstreamer0.10-plugins-bad gstreamer0.10-plugins-bad-multiverse gstreamer0.10-plugins-ugly gstreamer0.10-plugins-ugly-multiverse gstreamer0.10-plugins-base gstreamer0.10-plugins-good libdvdnav4 libdvdread4 libmp4v2-0 libxine1-ffmpeg ffmpeg flashplugin-nonfree sun-java6-fonts rar unrar p7zip-full p7zip-rar unace unp zip unzip ttf-mscorefonts-installer ttf-liberation mencoder mplayer sun-java6-plugin sun-java6-jre
2. Medibuntu is not ready for Maverick yet, but you can still play encrypted DVDs. Simply run the following command:
sudo /usr/share/doc/libdvdread4/install-css.sh
3. If you occasionally build applications from source or make your own deb files, here are some basic packages you should install:
sudo apt-get install build-essential automake make checkinstall dpatch patchutils autotools-dev debhelper quilt fakeroot xutils lintian cmake dh-make libtool autoconf git git-core subversion bzr
4. The following applications should really be default in Ubuntu but anyway, install VLC, OpenShot, GIMP (removed from the default installation a few releases back), gThumb, Pidgin, Skype, Adobe Air, Wine (ok, these last 2 shouldn't be in a default install), aptitude (removed from the default installation starting with Maverick) and Chromium using the following command:
sudo apt-get install vlc mozilla-plugin-vlc openshot gimp gthumb pidgin skype adobeair wine aptitude chromium-browser
Disclaimer / notes: This is just the basic stuff to install after a fresh Ubuntu 10.10 Maverick Meerkat installation and it includes a lot of non-free / restricted applications as well as some very popular non-default applications. If you'd find some applications which you've never heard of, it wouldn't be a "what to do immediately after installing ubuntu" post anymore but a regular post about new applications with a catchy title.