Blender is a 3D modeling, animating, rendering, post-production "killer" application, licensed under the GNU GPL. Since being set free from its proprietary license, Blender development has gained momentum, adding, among other things, video and audio sequencing features. Many free and commercial projects use blender for their 3D needs right now. Blender is also the the first tool of its kind with a built in gaming engine (this feature is supposed to have its comeback in the 2.33 release).
The recently released Gimp 2.x series follows the ever popular 1.x series, completely reconditioned and even caught up to the most expensive of commercial tools. The Gimp now supports CMYK color, has a great scalable vector graphics (SVG) path system, Python scripting, and windows you can dock however you like.
For the longest time people have said, "If you want to produce music, you should buy a Mac," but Linux now has comparable tools we can proudly display. Together with ALSA, the JACK server, and LADSPA plugins, Ardour should fill the gap for making professional music on Linux. A good article about this killer application and a few other mature music tools (which you can use with Ardour) can be found on OSNEWS.
A classic example where GNU/Linux lacked usability in the past is the realm of CD/DVD-Burning GUIs. K3B comes to the rescue and provides an interface that is at once clean, easy to use, and very powerful. If you need to burn a CD for a friend, say one that has a little movie and a program on the data track and an additional music track, and want it formatted to play in any hi-fi system, K3B simply works.
Having an integrated development kit is important for many developers. KDevelop 3, which had been written from scratch (not built on top of KDevelop 2), provides facilities for Ada, Bash, C/C++, Fortran, Haskell, Java, Pascal, Perl, PHP, Python, Ruby, and SQL programming and scripting. Moreover, it doesn't favor the programming of KDE applications as one might think; one can easily start a GTK project. For people who refuse to use KDE tools there is a similar powerful tool called Anjuta.
Proprietary page layout programs have every right to fear for their market share since Scribus 1.1.6, a GPLed product, has been released. This program not only fulfills all the features of its proprietary counterparts, including CMYK color, but also contains some unique features like SVG import and export.
The motion picture industry has driven the development of CinePaint, the Free Software painting and image retouching program designed with 35mm film in mind. It has been used to make such monumental films as The Last Samurai, Stuart Little, and many more.*
There are a few places where Free Software on GNU/Linux is still in need of work. Video editing is one of these. Though Cinepaint is a marvelous video touch-up tool, it is not built for all video editing purposes, and these other needs are not filled by any other notably "killer" Free Software tool. Despite how powerful and groundbreaking many of these programs are, some of them are not included by GNU/Linux distributions. Even when they are, there tends to be little, if any, documentation provided for them, sometimes even with distributions that package books along with their installation disks. What good is a killer application if you don't know how to use it?
But these days, if you need to get things done, there's generally a great piece of Free Software waiting for your purpose. It's just a matter of people noticing.
Thanks to my assistant Christopher Allan Webber, who helped greatly in the polishing of this article.
* Correction: Lord of the Rings was cited as having been made with CinePaint in an earlier draft of this article, but CinePaint developer Robin Rowe says, "When I asked Weta about their use of CinePaint the answer was frustrating.
They could neither confirm nor deny it," so we removed that mention from the list of film projects where we were sure CinePaint was used.