Bethesda has announced its plans to release the official toolkit for RAGE, which is coming to Steam sometime later today. The tools are the same ones used to create id Software's first person shooter RAGE and its recently released expansion pack, The Scorchers, both of which are powered by id Software's state of the art idTech 5 engine.
The toolkit contains everything including the world building tool, idStudio, as well as game maps, assets, and everything else you need to mod RAGE.
The toolkit weighs in at a heavy 35GB, so expect the downloads to take awhile once they go online. id Software has provided some reading material for prospective modders to go over in the meantime. These include a welcome document and a FAQ/EULA to answer any questions about their use.
Welcome to the Rage Toolkit
RAGE Toolkit FAQ and EULA
To download the toolkit, head on over to the Tools section of Steam and grab it there. Bethesda notes that it's the same place you'd go to download the Skyrim Creation Kit.
The toolkit comes with the following caveat, which you should look over—though it should be noted that it's no different from anything you might read on Skyrim's EULA:
"If You distribute or otherwise make available New Materials, You automatically grant to Bethesda Softworks the irrevocable, perpetual, royalty free, sublicensable right and license under all applicable copyrights and intellectual property rights laws to use, reproduce, modify, adapt, perform, display, distribute and otherwise exploit and/or dispose of the New Materials (or any part of the New Materials) in any way Bethesda Softworks, or its respective designee(s), sees fit."
