a real killer app in KDE is kdevelop imo. Sure, it's only usefull to developers but it's extremely easy to create a project with kdevelop. i'm not talking about wizards that'll provide code for u and that kinda stuff, but it takes care of setting up configure scripts and makefiles for instance. working with autoconf and automake to create configure scripts and makefiles is pretty annoying imo because it's not something u learn to do in a couple of minutes. Kdevelop however does it for u so all u need to do is code which is great cuz u don't waste time on makefiles and such.
Another great thing about kdevelop is some of the stuff u can see about ur project. for instance, if ur doing a c++ project in kde, there are views where u can see each class's methods, attributes and a lot of other neat stuff like that.
i had to use Microsoft Visual C++ at school last year, which also makes it possible for the developer to just create a project, code and not worry about anything else (apart from the damn thing crashing of course) and also provided neat views on classes and it's parts.
Well, Kdevelop is just as easy to use, if not easier than Visual C++, and it's totally free and open source.
if ur working on C/C++/QT/KDE projects, Kdevelop is something u should really check out and chances are u'll never want to go back to not using it again