From Ironic Software:
Start Yep for the first time and it automatically shows you all your PDFs, iWork and Office documents. The strength in Yep is being able to see your documents and search through them with out needing to worry where exactly you saved it to. Add your own tags to documents - which is like storing them in two places at once. Yep also has the ability to automatically file documents into a simple date based folder system located in your Documents folder. The result is that its easy to keep a running track record of receipts, invoices, research papers - without having to think about where to store it.