DesktopTwo (An Online Desktop)
I found out about Desktoptwo, an online desktop with a file manager, music player, and more; while checking out the Web 2.0 awards at SEOmoz.org.
Desktoptwo lets you sign in to an online operating system that looks like a normal computer desktop. It lets you store up to 1 Gb of files for free. (You can upgrade to more space for a price.) It has an MP3 player, a file browser, and yes you can even change your background.
I definately recommend taking a look at it! Check out Desktoptwo's website and if you are really curious you can check out another "web-based operating system" at EyeOS.com