I think TemaTres looks like a good candidate to manage a controlled vocabulary. It’s open source, web-based, and runs on PHP/MySQL. I spent some time using it and it looks like a nice, lightweight manager. It’s pretty good out of the box, and with a little bit of effort we could customize to our liking.
The project homepage: http://www.vocabularyserver.com/
Here’s a favorable review of the software: http://databits.lternet.edu/spring-2011/managing-controlled-vocabularies-tematres
I created a demo installation to give it a try at:
Controls to add/modify terms appear as a dropdown in the top menu under the label “Menu”