Amazon gives you lots of free storage, 20GBs, but you can only get it to through your Web browser and you need to use Flash to upload files. I find it clumsy.
Eventually Apple's iCloud will be fully integrated into iOS and Mac OS X, but for now it's still looks and feels like an add-on. Even now you can only use it to its fullest advantage if you're running Mac OS X Lion.
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.
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.