Windows Live SkyDrive is now open to the public in beta form.  From what I can tell, it is basically 500 megs of drive space on the web that you can use to store personal documents or things you want to share.  This isn't the first time something like this has been done.  I can think of older stuff like xdrive, but this is the first time I recall a major player doing it.  It also has very slick integration with the system.  With the installation of one activex control, you can drag and drop things between your computer and the web.  Quite cool.