Chosen and Select2 are the two more popular libraries for extending selectboxes.
Both seem to be actively maintained, Chosen is older and supports both jQuery and Prototype.
Select2 is jQuery only, its documentation says Select2 is inspired by Chosen, but doesn't detail any improvements made (if any) or other reasons for the rewrite.
Two libraries have pretty much same feature set, the only comparison I've found is a somewhat inconclusive jsperf test page.
Does any of these libraries have any advantages over the other?