Script.aculo.us is a JavaScript framework that allows you to add effects and advanced JavaScript features to any web page. Although Script.aculo.us can be a little slower than Mootools on occasion it makes up for this by having more stuff built into the library components. For example, the slider control is available of both frameworks, but the Script.aculo.us version comes with some handy features (like filling one part of the slider) where the Mootools version doesn't.
My one problem with Script.aculo.us is the website. There are a few examples and descriptions, but most of what you need to know about seems to be poorly documented or it hasn't been updated for many months. There is a forum where people can put questions and offer solutions, but again, this seems to be very much out of date and any solutions offered seem to only work with older versions of the framework.
Overall a good framework, with a poor site.