Tag2Find lets you tag all your files - images, docs, music etc - and then search files by tag.
Here's méprisant's comments:
"All of us understand the benefits of tagging. Gmail calls tags labels. Flickr uses them, as does every other Web 2.0 startup. Tag2Find brings tagging to the operating system itself. As far as I know, every file can be tagged. Tag2Find stores the tag with the file itself, and in an internal database. Files retain their tags, come what may (presuming of course that they are not deleted)."
Tag2Find : OS Level Tagging