At the moment, I’m using the Rico library for effects; I don’t use “AJAX”, Drag and Drop, LiveGrid, etc. On Evolution we use the Rico library with the extra features we don’t use removed. The resulting library is 38KB plus an extra 29KB for Prototype. It seems like moo.fx can do quite a lot of what Rico can do and there is an Accordion-like effect demonstrated on the moo.fx homepage.
moo.fx also claims better Opera and Safari support which is also a bonus. I’m not sure whether I have the energy to migrate Evolution to use moo.fx but it’s certainly a library I’d look at for further use in any similar projects. It’s also open source and released under the MIT license :)