I liked Gmail originally because it used tabs. I have seldom had an email that was permanently tagged with more than one tag, but I have imagined having files in a filesystem with multiple tags. Bloggers commonly use tags but I don't know if they're successful.
On a filesystem where tags were not native, you could implement tags as subdirectories, using symlinks to achieve that the same file had multiple paths giraffes/history.html and history/giraffes.html.