Benubird PDF (free) is a great idea for document management, but it's marred by some bugs and strange design choices. What Benubird PDF does is take all of the documents you have scattered over your hard disk (you tell it which folders to search in) and then it copies all of them to a single directory, gives them unique names (so your 500 copies of "readme.txt" become 0001_readme.txt, 0002_readme.txt, and so on), and then allows you to sort and search them by file properties such as author, creation date, subject, etc. (Many files, especially .doc and .pdf, have these properties but they're rarely displayed in Explorer unless you change default settings.) You can add categories to files, and you can create folders which will sort files according to various rules--and a file can easily exist in multiple folders, as they're categorical tools, not physical disk entities. It sounds great, but the implementation is lacking.
First, Benubird PDF doesn't create references to your documents...it copies your documents. This means that if you have, say, 100 gig of document files you want Benubird PDF to manage, you need an additional 100 gig of disk space. Second, this process is slooooow. My Documents folder contains some 60K files, and even limiting Benubird to just the file types I wanted to manage (PDF, DOC, TXT, XLS, and the like) meant waiting a long, long, time for it to copy them. I eventually aborted the process after about 5000 files had been copied, but this was enough for me to test its features.
For everything I liked, there was a "gotcha." Hey, cool, I can filter my file list easily and then add properties. Hey, uncool, everytime I bring up a dialog box to work with one of my filtered files, Benubird PDF removes the filter so I'm back to looking at the full list. Cool, Benubird PDF will "watch" a folder and, when a file of a particular type is added to that folder, it will add it to its library. Uncool, it flags the file as missing and no matter how often you point it back to the source file, it still claims the file isn't there. (It was. I checked.) Last, and this really isn't a critique of Benubird PDF itself: To make optimal use of it, you must add categories to your files. This takes a long time if you've got, say, 60,000 of them.
Benubird PDF is free, which is good. There is a "Deluxe" non-free version forthcoming, but it has not yet been released. Hopefully, by the time they ask people to pay for this, the kinks will be worked out.