Many tools use some kind of tag-normalizations. This seems to be a fundamental part of each tagging engine wich should probably be copied by OSSTag.
Only three tools go beyond a flat tagging structure:
No tool made any attempt to use some special thesaurus for tagging. Some really good ideas can be found here: http://www.groklaw.net/article.php?story=2006052522234947
A more concrete way to use a special thesaurus with information from Wikipedia is described at Which tags do we allow
This is also a way we could use to provide a central tagging server
No tool implements distributed tagging-engines. All use a central database.
The tool that is closest to what we need would be FreeTag .
I suggest to write a prototype from scratch with the primary goal to try out several tagging-structures. OSSTag v0.1 could be enhanced for 3-level-tagging and n-level-tagging in combination with wikipedia or ACM thesaurus. Then we could try to tag a project with each combination of these ways and see which fits best for our needs ..