By no means an exhaustive list, but here's a few we came across..
Our Experiences So Far..
So far we've used the MooTools version of TextboxList - not only do the folks at DevThought have one of my favourite looking websites, but they've also come up with a good implementation of the concept. It worked out of the box - in all it took us about 10 minutes to get it in place as required.
We also started to look at Proto!Multiselect - initially it looked as if it hadn't been updated in a while - reading around on various blogs we found a number of people had performed fixes and updates, but couldn't find a centrally updated copy. Enter, GitHub - we took a copy of the original InterRiders script and applied a number of patches from the community and put it all together into it's own repository - I can't take credit for anything here other than pulling together the various updates into one version. We haven't got round to using it yet so I suspect there will be fixes to be made. All the changes are credited to their authors - so if you spot some i've missed then let me know!
I'd planned to go through a few examples of Rails integration but i'm out of time so will have to save that for another post..