Taggee is an open source tagging server written in ruby that uses MySql for storage. It is highly scalable and lightweight.
The first early version is now available for testing. It is fully functional and can be easily tested.
Downloads are available at http://sourceforge.net/project/showfiles.php?group_id=161280 with information about additional requirements (MySql, ruby and mysql-ruby) and how to run is available at http://sourceforge.net/docman/display_doc.php?docid=32042&group_id=161280,