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Like iCloud, SkyDrive will eventually—say Windows 8—be integrated into the operating system. For now, you need to use it with a Web browser. On the other hand, with 25GBs of storage, it offers the most free storage of any of the services.
Don't think UbuntuOne is only for Ubuntu Linux users. It works with Windows as well. Like Dropbox, UbuntuOne already integrated with both Ubuntu and Windows file managers. On Windows 7, however, it can still be very slow at times. In my experience, however, it does fine with Windows XP.