This small extension to Share provides a new site dashlet to display the most popular tags within the site as a tag cloud.

The dashlet uses the existing tagging REST API, so no additional repository tier scripts are required. The zipped files can be extracted into your tomcat/shared/classes directory to add the Share support. You should find that the directory structure will be automatically created, although you will need to move the site-tags.css file to the components/dashlets directory in the webapp.
It should be trivial to write a second dashlet to surface a global tag cloud, suitable for display in a user dashboard, using the list all tags method, rather than the tagscope-based approach used by this script.
UPDATE: The latest version of the code can now be found on the site tags dashlet page.