There is a catch, however. These tools are based on the ones Valve used to create the game itself, and as soon as you launch the app it's obvious: Portal 2 Authoring Tools are not for casual users. It includes the Hammer World Editor, which allows you to create levels or edit current ones. There is also a Model Viewer and Face Poser. It's pretty complicated stuff, but if you've got the time and dedication, there is a good 'Getting Started' guide, and some example content.
Portal 2 Authoring Tools are PC only, and require a copy of the Portal 2 to download and use. It will be interesting to see what people create!