The filesystem sucks. Or rather, the filesystem doesn't suck, but it's not good enough to usefully interact with documents -- that's why iTunes and iPhoto implement their own document libraries. Of course there are so many other useful ways to look at your documents. Decent, live, dynamic search functions would help. As would something like Autonomy built in to the OS, or versioning, or something else other than having documents in one place only with no way of figuring out what they are other than a name. Anyway, rant over.
Six Degrees from Creo is a piece of software that says it'll add [a very small part of] this missing functionality to your filesystem. Macfixit explained it thus: "It is constantly working in the background building and recording relationships among your files. So for example, if a friend (we'll call her Sally) mails you a jpeg as an email attachment, it will store this relationship. Suppose you move the file from the email attachment field and place it in a folder in your Documents folder. Now suppose that weeks later you want to see that file but can't remember where you stored it. You don't even recall its name. All you can remember is that Sally sent it to you. You can launch Six Degrees, type in Sally, and it will show all files that are related to Sally. From the list, you will likely recognize the file you are looking for.
It's not perfect. It's not integrated. It's just a fancy search engine. But it's a start, and that's good enough.