The first, most popular one is the venerable scriptaculous. It does require another, lower-level library called Prototype, which is a heavy download at 140k. BUT someone has done the web a favor and managed to super-ultra-compress it down to 14k, which is quite acceptable.
Scriptaculous is very popular, and lots of people have written stuff with it that is available for anyone to use – check out this set of awesome tooltips, for example. But it does pretty much anything we’d need out of the box, and it’s very well documented.
The second candidate is jQuery UI, which is built on the other major JS framework, jQuery. I haven’t used jQuery UI, but it seems to do about the same stuff as scriptaculous, and it is also popular
enough to have some free extensions / implementations,
like these tooltips.
And then there is YUI, which is a standalone UI library. It is also in wide use and does pretty much anything we’d need.
My recommendation would be that whoever is building the front end code should pick whichever one they’ve worked with before, to minimize dev time. If all three are unfamiliar, I guess I’d recommend Scriptaculous because I’ve used it and I remember it being pretty straightforward.