The KDE Homepage
In addition to having all sorts of user information, the KDE homepage also has
things like how to get the current development branch via cvsup, mailing lists
you will want to join, etc...
KDE Developers Corner (developer.kde.org)
Here you will find everything you need to begin KDE programming. KDE docs, Qt
docs, 7 tutorials, etc... A must read site before you start coding.
Qt 2.0 API documentation and tutorial
KDE uses Qt as a core library, so it's important to know how to use it first.
Luckily, Qt comes with excellent tutorials that both myself and others have
even used to help teach C++ before. Along with that, you get a full set of
example applications and the API docs you will need to learn the Qt classes.
This documentation also comes when you download Qt, so you can just open up
html/index.html where you untarred the source code to browse the docs locally.
KDE 2.0 API Documentation
Like most stuff, this is also hosted at developer.kde.org. KDE extensively
uses an inline source code documentation processor called "KDoc", and here
you can view or download all the API documentation in HTML.
Antonio Larrosa's KDE2.0 tutorial
Antonio has written an eight application tutorial, the last two of which
feature a full web browser. Available in both English and Spanish.
KDE Web CVS
Browse the latest CVS commits to KDE2.0. Also interesting is the CVS commits
list archive.