The official Witcher 3 ModKit has been released by CD Projekt RED and you can download it from Nexus Mods, which is the official host.
Can we get a hell yeah?
CDP spoke with Gamespot recently regarding the ModKit and there is a nice Q&A posted on the site.
“The modding tools we are releasing are not an editor,” said CDP. They’re meant for people who are somewhat experienced with modding software and are designed to make creating mods easier. They give access to game files and allow for easy extracting, compilation, and decompilation of resources. Beside visual changes, like modifying textures and models, they will allow to change UI assets and script files. The majority of gameplay-related mechanics are written in scripts.”
The developer said it is also talking with Sony and Microsoft regarding the possibility of console players being able to access mods, but there is nothing to announce on that end as of yet.