jQuery is so mainstream and popular that other Javascript frameworks are often forgotten. There is Prototype, dojo and more. In the early days of Javascript frameworks MooTools was front runner.
MooTools is a compact, modular, Object-Oriented JavaScript framework designed for the intermediate to advanced JavaScript developer. It allows you to write powerful, flexible, and cross-browser code with its elegant, well documented, and coherent API.
As you can read from their site, MooTools is more for advanced Javascript coders. It has a more steep leaning curve than jQuery, while jQuery is easier to implement without much knowledge of Javascipt.
Javascript language was never a favorite of mine, I learned it at college before there were frameworks. Maybe it was the way they taught it that created my distaste for the language. The exam was with pen and paper instead of a computer, so we had to debug the code ourselves.
But those days are long gone, in between jQuery and MooTools there is the right framework for everyone. If you are handy with Javascript or want to be able to do more, you should give MooTools a try.
I selected nine MooTools scripts, many have their equivalent in jQuery but the MooTools versions are more versatile if you know your code.
Hat tip: Joomla! uses MooTools.