REDkit was a modding tool for The Witcher 2: Assassins of Kings developed by CD Projekt RED that was released to the public on 13 May 2013. It allows users to create new lands, quests, and NPCs for the game. For one month prior to the release date, REDkit was in closed beta.
REDkit aimed to provide a very professional modding experience by what has been seen of it so far and an example of the sort of experience it can create can be seen in the almost completed mod 'Lykaon'.
Sometime after March 2016, CD Projekt removed the download links and re-directed the REDkit page to their main page. There's no word on when, or if, they'll bring it back.