This is exciting - I hate discovering stuff like this at work, I just want to go home and install it now :) Phil might like this... It's called segusoLand, but I'd suggest looking through the info and screenshots for a straightforward look at how it works.
I really like the way its "categories" and database-backed, so if you used it all the time, you wouldn't need file-structure-level folders, really (apart from when using other apps?). I also suspect the "do things in the future" ease-of-use could throw up some interesting things to do. It also runs on Linux :) (Can't work out the window theme - looks like the KDE Plastik theme, but the apps are all gnome-based. Must be a similar thing on Gnome.)
It'd be great if it tied in with apps, so that you could choose files from within an app using this, with the appropriate files selected according to what the app is, but I don't expect that for a few years after widespread adoption ;)
I'm also hoping it has quick keyboard access - being able to type the filename in - or better, part of a filename - would be fantastic.
Thanks to nooface.