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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.