1. BasKet - beautiful and powerful notes-taking application, in my opinion BasKet deserves the name of the best notes-taking application for Linux.
2. Krusader - very powerful file manager for KDE3 with an interface similar with the one of Midnight Commander. Krusader 2.0.0-beta1 for KDE4 was released recently.
3. K9Copy - DVD ripper for KDE with advanced options. You can read a full review here.
4. KWord - KWord is the word processor application included in the KOffice suite. It's lighter than OpenOffice.org, yet offering many options and support for the OpenDocument format. Work is in progress for the KDE4 port too.
5. KDiff3 - a file differences and comparing tool.
6. Konversation - currently at version 1.0.1 for KDE3, Konversation is a very user-friendly IRC client with lots of options, highly configurable, but lacking event-based scripting. Konversation 1.1 for KDE4 is in the works.
7. kdetv - KDE TV viewer with support for Teletext. (Download from here if official website is down)
Official website | Official website
8. Krita - Krita is the image editor included in KOffice. Although not as popular as the GIMP, Krita includes many effects and several powerful tools for image manipulation.
9. KPlayer - powerful KDE video player based on MPlayer, with support for subtitles and TV.
10. KMyMoney - personal finance manager for KDE including most of the powerful features needed for managing your finances.