In the Mac’s Finder (the equivalent of Explorer, for you windows people), a window looks something like this:
The area on the right displays files and folders in the current folder. The area in the upper left (above the line) lists all of the available disks on the system. The area in the lower left (‘Desktop’ and below) is the sidebar.
The sidebar is nice, because you can drag discs, folders, and even individual files there. If you click on a folder, the finder changes into that directory. If you click on a file, it will open in the appropriate application. To remove an item, drag it out of the window and it disappears with a poof — literally: there’s an animation and a sound effect that go with it, same as removing an item from the dock.
It would be really handy if it was possible to have contextual items on the sidebar. If you’ve selected an MP3 or WAV file on the right-hand window, your designated ‘music’ folder should appear in the sidebar; possibly iTunes should appear there as well. There can be a neat fade-in effect, as in iChat when contacts come online.
If you select an AVI file, a ‘television’ or ‘movies’ directory could show in the sidebar, along with a video player. (I use vlc, but presumably QuickTime would be the default.) Drag a file to the folder to move it to that directory; drag it to the player, and it will open.
This should be relatively easy to implement, its interface shouldn’t be too complicated, and it would be one step closer to having the computer think for you.