CD Projekt RED's upcoming REDkit is the Witcher developer's modding tool which will allow fantasists to create their very own RPG worlds (or tinker with the Witcher 2 world itself) using the REDengine. The modding tool will be free to owners of The Witcher 2, and will be released during the first half of 2013.

The REDkit closed beta currently has over a 100 modding teams providing feedback, so unless you're part of the beta testing you will have to wait a while, but, here's a time lapse video from CD Projekt RED that showcases the creation of a brand new adventure!