I've looked at:
- https://github.com/gfranko/jquery.selectBoxIt.js (last release 10/2013)
- https://github.com/select2/select2 (last release 11/2017)
- https://github.com/harvesthq/chosen (last release 6/2018)
- https://jqueryui.com/selectmenu (last release 9/2016)
All of these have their pros and cons - chosen is probably the most actively maintained. At this point I'm looking for any other suggestions from developers who've had first hand experience with other plug-ins like these.