Tagging software anyone?

I'm looking for a simple and useful software. I need to archive lots of documents (PDFs mostly) under multiple categories or 'Tags'. For instance, a document on Indian history should be placed both in "History" and "India". Users of Gmail would understand what I mean. Normal folders won't work because one needs to store multiple copies of the same file under each folder. I could think of one simple way out: Store the actual files in a master folder, and place the appropriate soft links in each category folder. This way, one would get a nice file view (as though the files are actually placed in that folder), while each file is stored only once. But how will I search for files common to both tags? I can always write a script to do all this, but why bother if someone has done it before? I actually found couple of applications that do exactly what I want, but they are all paid software or beta versions (with a disclaimer on stability!!).