Punakea. Tagging for Mac OS X.
Punakea is a Mac App designed to help you cope with the day-to-day struggle of managing your files. Complementing Spotlight, it allows you to tag your files and bookmarks, freeing you of the strict hierarchy of the Finder's folder structure. It's like Finder Tags, but with a tag cloud!
Please note that the code is not in the best shape and we do not have much time to work on Punakea at the moment - if you want to take over maintaining Punakea, please drop us a note!
The official web page is hosted here.
More information can be found in the wiki.