Amarok - Audio Player
Linux has plenty of audio players, like the eternal XMMS or the newer ones Banshee, Exaile or Audacious, but I always prefer KDE applications over the ones made in GTK. But for me Amarok wins not because it's a KDE application, but because it definitely is the most complete player out there for Linux. So Amarok is my audio player of choice, due to its rich features like the well-implemented collection management system, the Last.fm song submission or the possibility to expand its functionality through many scripts.
XChat - IRC Client
Since IRC is a great protocol for text-based chat, and it's around for almost 20 years now, it is still used at large scale by the Linux community and networks like Freenode or OFTC have thousands of users at any given time. This is the reason why I considered to include an IRC client for chat purposes rather than an IM client for protocols like Yahoo! or ICQ. XChat is my choice, and the reasons are numerous: simple and clean interface, extremely powerful scripting and plugins interfaces (including Perl, Python and C languages as default), helpful forum.
Kate - Text Editor/Programming IDE
Even though some probably don't consider it a complex IDE like KDevelop or Eclipse, Kate is a fast text editor which can be used for taking notes or development of smaller projects. It includes a spellchecker, highlighting for many languages and many other features. Kate is shipped by default with any KDE-based distribution.
GIMP - Image Editor
I can say GIMP is one of the projects which really receive a lot of attention. Even the official website has a nice layout, looking professional and clean. It is undoubtly the best image editor the Linux community has at the moment. It also has complete documentation.
Gwenview - Image Viewer
Currently at version 1.4.2 for KDE 3, Gwenview is light and fast, but its functionality can be expanded through many plugins. It has a preview mode together with a file browser, an option to view all the images in a directory as a presentation (the Slideshow option), and you can make even basic image editing with it.
KTorrent - BitTorrent Client
Sharing files in a community which is based on open-source software is definitely a need, and what better protocol can be used except BitTorrent? Even though it was developed after 2000, thus being relatively new, BitTorrent is in a continuous growth and all the big distributions offer their OS through BitTorrent. Just do a search on 'Ubuntu' or 'Fedora Core' on any big torrent site. KTorrent is a complex BitTorrent client, capable of showing many information like peers, chunks, or trackers. It also offers the very useful option to only download specific files included in a torrent, not all of them. You can also use KTorrent to create your own torrents and start seeding.
Firefox - Internet Browser
Being the most popular browser on the Linux platform, Firefox was always a secure Internet browser. It can be expanded through many add-ons, it supports themes, it shows a web page accurately where other browsers fail to do so. Most people are complaining that it's slow (and it is in my opinion), but it seems the best web browser out there. And, I have to mention it, it's the biggest competitor to Internet Explorer on the Windows platform.
Konsole - Shell-like Application
Since it's well known that doing many things is ten times faster with the console, I always have one opened and docked with KDocker in the system tray. Konsole allows you to customise it in many ways, from the background and font selection to font encoding or the possibility to set bookmarks (not that this one specifically is very useful). It also features tabs, allowing you to open several sessions in the same Konsole application.
Kaffeine - Video Player
Probably here are better applications (or at least more popular) than Kaffeine, like VLC or MPlayer, but I always preferred this one. Even though each release has some annoying bug or another, it plays well DVDs, has subtitle support, includes a playlist and can be docked into the system tray. It can also play network streams. Overall, it's small and lightweight yet still provides the user with the necessary features a video player should have.
TVTime - TV-Tunner Software
For those of us having a TV-Tunner, an application like TVTime presents a good and fast way to watch TV on our PCs. It's true, it does not have advanced options like recording or PiP, but at least you can watch TV. The keyboard shortcuts are simple and intuitive, you can take screenshots and, one nice thing, in the title bar always appears some funny quote.
I know this list is not complete. It cannot be: it only contains ten applications which I could not live without; I know I have left out IM clients, office suites, file managers and so on.
Updated: May 24, 2008 (Created: May 24, 2008)