I don't understand where the plugins central is: on Omeka.org or on github.com/omeka? There are well categorized on Omeka.org, but we can find a lot more on github, often fresher. What is the best to use?
In fact, I'm looking for a plugin that manages taxonomy (an hierarchised vocabulary, not simple tags). Where should I search?
Infodoc & Knowledge management
http://github.com/omeka is the where the ongoing development of the plugins and themes the Omeka team develops happens, plus Omeka itself.
The official home for released add-ons from both the Omeka team and other developers is http://omeka.org/add-ons
Omeka.org is "plugins central:" it's where the official releases, pre-zipped in the right format, go and where you can get plugins developed by third parties. If you're seeing actual "fresher" releases of plugins from Github that aren't on omeka.org, that's likely either unreleased code or a simple oversight.