GIMP - powerful image editor
Also known as the GNU Image Manipulation Program, GIMP is a powerful image editor using GTK and providing the necessary tools and effects for complete image manipulation. Documentation is also abundant, and the web is full of tutorials and tips on how to use it. This is by far the most complete image editor for Linux, and it's licensed under the GPL. If you are looking for a KDE editor, you may also want to try Krita, included in the KOffice suite.
Scribus - desktop publishing application
Scribus is a complete desktop publishing application built in Qt 3. It allows publishing in PDF and provides advanced tools, being completely free of cost. It also allows creation of SVG images. The last version is 22.214.171.124.
Blender - 3D animation
Blender became more and more known lately due to the free movies which were developed using it, Elephants Dream and Big Buck Bunny. A game based on the characters in Big Buck Bunny is also on its way. Blender's interface is implemented in OpenGL and it has a nice, pleasant futuristic design. It provides 3D modelling, animations, rendering and many other features.
OpenOffice.org - office suite
OpenOffice is the most popular free office suite for Linux. It's very powerful, feature-complete, it supports the OpenDocument standard and is the main rival for Microsoft Office on the Windows platform. Although slow and pretty much bloated, OpenOffice is definitely the office suite of choice for a person who creates advanced documents.
Audacity - sound manipulation program
Audacity is a sound manipulation program which includes many effects like the noise removal tool (default values provided offer great results too), possibility to save as FLAC, OGG Vorbis, MP3, import/export files, record sound and many other features. I tested it recording an old cassette with Bob Marley's Birth of a Legend album, and the noise removal tool really impressed me. For audio manipulation, Audacity is the best in my opinion.
Cinelerra - video manipulation application
Although it has a little unusual interface which might be confusing at first, once you get used to it, Cinelerra becomes a powerful tool for manipulation of both audio and video.
Emacs - integrated development environment
There are very powerful IDEs out there, like KDevelop, Eclipse or Anjuta, but I decided upon Emacs since it's one of the most popular, and probably already a symbol for most programmers out there. It runs in both CLI and graphical mode, it includes highlighting for many languages, extremely powerful configuration options, good indentation support, comprehensive help, Lisp scripting. These are just a few. Emacs still is one of the most popular IDEs, and once you get to know how to work with it, it becomes faster than any other application for programming out there.
Inkscape - vector graphics application
Inkscape is a vector-graphics application which allows saving in PDF, SVG, PostScript and several other formats. It's written in GTK and provides many tools and effects.
K3b - CD/DVD burning application
This is a must-have tool in order to write CD/DVDs, audio CDs or burn CD images. It is the most complete KDE application for doing it. Two alternatives I can think of is Brasero or the closed-source Nero for Linux (which offers only a trial version anyway), but neither of them look to me as friendly and feature-complete as K3b.
Updated: Jul 06, 2008 (Created: Jul 06, 2008)