Here are guides to some of the software applications available with Ubuntu. The structure of this list follows the Gnome desktop's menu structure.
Accessories - A quick look at the variety of software accessories in Ubuntu.
Games - GNU/Linux games and links on how to run Windows games in Ubuntu.
Graphics - An overview of the graphics software available.
Internet - Ubuntu's internet applications and information on networking.
Office - A quick tour of the default office programs, and some useful extras.
Other - Software that doesn't fit well in the other categories.
Programming - Programming text editors and some development environments.
Sound and Video - Aplications for music, video, ripping and burning.
System Tools - Programs that deal with your hardware and software system.
Preferences - Taking a look at some of the key preferences applications.
Administration - A list of some of the applications under the 'Administration' menu.
Help - Sources, offline and online, for help and support.
Best of the Rest
List of additional software recommended by users and available in the official repositories.
Firestarter - Tighten or loosen your Internet security.
GdMap - Analyse disk use via visual 'treemaps'.
Kdissert - Mindmapping software great for brainstorming for essays and projects.
Revelation - Password Manager.
Further Reading on Software
Sites that help in assessing software, though some of the listed applications are not available in the repositories, meaning that they cannot be installed with applications like Synaptic Package Manager.
GnomeFiles - Categorizes, rates and lists Gnome-based software.
KDE-APPS.org - Categorizes, rates, reviews and lists KDE-based software.
osalt.com - Lists 'open-source' alternatives to proprietary software.