[edit] Marble's Secrets

A good introduction to get started is:

Marble's Secrets Part I: Behind the Scenes of Marble...

http://www.kdedevelopers.org/node/3269

Marble's Secrets Part II - Walking In The Shoes Of Slartibartfast ...

http://www.kdedevelopers.org/node/3272

Marble's Secrets Part III: The Earth in a Download

http://www.kdedevelopers.org/node/3275

Marble's Secrets Part IV: Run Marble, Run!

http://www.kdedevelopers.org/node/3769