If you want support for MP3 files, Youtube video, and Java on your webpages, you could hunt down several packages - or just install the ubuntu-restricted-extras package install all the codecs and other files in one step. This also includes Microsoft core fonts.

sudo apt-get install ubuntu-restricted-extras

Eliminate Irritating Update Notifier Pop-Unders

This from Joseph Sinclair

In Ubuntu 9.04, update-notifier doesn't display an icon, it actually runs update-manager full-screen as a "pop-under". It's easy to miss, and there's no way to make it NOT run (so on a laptop, for instance, where stupid useless no-change updates are pending, you'll get the blasted thing running every time you boot, and quite often multiple times in a session).

There is a "magic" command to make it stop and go back to how it used to run (which you may have to run regularly since some updates seem to overwrite it), but it must be run for every user who can run updates:

gconftool -s --type bool /apps/update-notifier/auto_launch false

Adding that to /etc/bash.bashrc seems to be a quick-and-dirty fix that restores it for every user, and resets it if it gets overwritten. There's no guarantee this will work after the 9.10 update, but at least it works for now.

Additional Repositories

We are going to add the Ubuntu Partner and Medibuntu repositories which contain a number of apps.

From a terminal window, edit /etc/apt/sources.list...

sudo gedit /etc/apt/sources.list

... and enable the intrepid partner repository:

[...]
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository. This software is not part of Ubuntu, but is
## offered by Canonical and the respective vendors as a service to Ubuntu
## users.
deb http://archive.canonical.com/ubuntu intrepid partner
deb-src http://archive.canonical.com/ubuntu intrepid partner
[...]

Then save the file.

To enable the Medibuntu repository, please do the following:

Import the repository:

sudo wget http://www.medibuntu.org/sources.list.d/intrepid.list -O /etc/apt/sources.list.d/medibuntu.list

Import the gpg-key and update your package-list:

sudo apt-get update && sudo apt-get install medibuntu-keyring && sudo apt-get update

Install software

Yes this all goes on one big long line --

sudo apt-get install amarok flashplugin-nonfree audacity azureus 
banshee bluefish dvdrip filezilla msttcorefonts gnucash gstreamer* gtkpod-aac 
sun-java6-bin sun-java6-javadb sun-java6-jdk sun-java6-jre sun-java6-plugin 
k3b kino mplayer mozilla-mplayer quanta kompozer scribus xchat-gnome 
bittornado bittornado-gui sound-juicer helix-player mozilla-helix-player 
googleearth acroread mozilla-acroread non-free-codecs ubuntu-restricted-extras 
libdvdcss2 opera xine-ui xine-plugin xmms2 xmms2tray xmms2-plugin-airplay 
xmms2-plugin-alsa xmms2-plugin-ao xmms2-plugin-asf xmms2-plugin-asx 
xmms2-plugin-avcodec xmms2-plugin-cdda xmms2-plugin-cue 
xmms2-plugin-curl xmms2-plugin-daap xmms2-plugin-faad xmms2-plugin-flac 
xmms2-plugin-gme xmms2-plugin-gvfs xmms2-plugin-ices 
xmms2-plugin-icymetaint xmms2-plugin-id3v2 xmms2-plugin-jack 
xmms2-plugin-karaoke xmms2-plugin-lastfm xmms2-plugin-m3u 
xmms2-plugin-mad xmms2-plugin-mms xmms2-plugin-modplug 
xmms2-plugin-mp4 xmms2-plugin-musepack xmms2-plugin-normalize 
xmms2-plugin-ofa xmms2-plugin-oss xmms2-plugin-pls xmms2-plugin-pulse 
xmms2-plugin-rss xmms2-plugin-sid xmms2-plugin-smb xmms2-plugin-speex 
xmms2-plugin-vocoder xmms2-plugin-vorbis xmms2-plugin-wma 
xmms2-plugin-xml xmms2-plugin-xspf vlc vlc-data vlc-dbg vlc-nox 
vlc-plugin-arts vlc-plugin-esd vlc-plugin-ggi vlc-plugin-jack vlc-plugin-pulse 
vlc-plugin-sdl vlc-plugin-svgalib thunderbird skype

You could also include

amule

A more detailed explanation is on HowtoForge.