The German software manufactory Soma-Zone has recently released Ammonite, a stand-alone third-party addition to DEVONthink Pro (and Pro Office, of course) that shows a tag cloud for any open database in a floating panel. From their web site:
Ammonite visualizes the tags used in a database and finds documents by including or excluding them from your search. Tags applied to many documents will appear larger then tags applied to a few documents. This helps you to drill down on the information you are looking for. You can limit the tags being displayed with the tag filter field in real time.
Read more on the Soma-Zone web site.
Thank you, Robby, for this cool piece of software!
Disclaimer: DEVONtechnologies is neither responsible for the operational reliability nor the overall quality of Ammonite, so download and use at your own risk.