Is there any program for tagging files in the computer?

On the web, we have a lot of websites such as flickr, del.icio.us which let us "tag" any url or photos that we shared. But I wonder if there's any program that let me tag the files in my own computer, so that I can search them easily by keywords, or through a tag cloud?
Google desktop makes it very convenient to search files, yet it doesn't provide the tag option. I found it more and more difficult to sort things into different folders, when there are more and more files in my computer. Perhaps "tag" would be a much more convenient way than "folder".