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While you can use Dropbox with a Web browser, it's best feature is that—no matter what operating system you use—it simply acts like another network drive. That's very handy.
There's no “G-Drive” yet, but Google offers bits and pieces of personal storage with its Google Play music service. As such, it's a great music player, but it's still no replacement for Dropbox.
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.