Amarok - in my opinion, this is the greatest music player and collection management for the Linux platform. I think it's also a killer application for Linux with all the features it has.
SMPlayer - one of the most powerful video players for Linux using the mplayer engine, and including support for DVDs, DVD ISO images, Matroska video files and much, much more. A must-have.
KTorrent - for me this is the BitTorrent client of choice. It has all the features I need, including the possibility to only download certain files in torrents, a feature which some other clients lack.
XChat - I think there is no other (graphical) client for IRC as powerful as XChat. Maybe KVirc, but regarding the scripting/plugins part, XChat wins over Konversation or KSirc easily. It supports scripting in Perl and Python by default, C plugins, and a Tcl scripting plugin is also available, but not by default. It can also be easily customisable via the /set variables.
BasKet - BasKet is a notes-taking application which I also reviewed in the past; it has a wonderful approach of letting you make full projects, not only simple notes, and it organises your data in baskets and sub-baskets. Except for the fact that the current version does not support printing (well, you can always export notes to an HTML file and print that one) it is definitely full-featured. I strongly recommend this application over other notes-taking applications.
Wesnoth - this is a turn-based strategy game, well-maintained, with a strong community and a beautiful gameplay, both in single-player mode and online. I must say, I love this game and I really appreciate all the work it has been put for creating and maintaining it.
K3b - definitely the most full-featured CD/DVD burning application, and it allows to create ISO images too. No KDE 4 port yet though, and it looks like development is currently stalled.
Emacs - I find Emacs to be an essential tool and one of the most powerful and popular IDEs out there.
Yakuake - a terminal application which acts just like consoles in FPS games (e.g. Quake, ET, UT). Yakuake can be hidden or shown using the F12 keyboard shortcut.
TVTime - this application is useful to those who own a TV card. In the past I also tried kdetv, but nothing compares with TVTime in my opinion.