I wrote a Gallery2 module that depends on the Tags module, called Tag Tree, (http://chad.parry.org/software/gallery2/tagtree.html). In the process, I also made some local changes to the Tags module, so that it would publish an event whenever a tag is updated. Tags is a great module and extending it to publish events will make it easier for other modules to build on top of it.
Patch against Gallery 2.3 RC1