All users should download MooTools from MooTools.net. We use github to develop the source code and it may not be stable at any given time.
Who are these crazy MooTools Contributors?
Check out some fancy Demos!
If you're going to try developing on mootools, start by forking the project from the project page. This will allow you to submit pull requests if you want your changes to be added to core: git clone firstname.lastname@example.org:YOURFORK/mootools-core.git
Enter the mootools directory: cd mootools-core
Next, get the dependencies, which will allow you to use the mootools build scripts $ git submodule update --init
Now you're ready to start. Create a copy of mootools with all the parts you want (as specified in package.yml) $ ./build > mootools.js
And you should be on your way!