Put simply, Dropbox is an amazingly useful combination of a Web service and a Mac OS X program that work together to make your data accessible from anywhere and to keep it synchronized between your computers
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If this sounds a bit like the iDisk feature of Apple’s MobileMe service, that’s because it’s similar…except that Dropbox is fast and reliable. Dropbox is also more secure than iDisk, as files stored on the Dropbox servers are encrypted and are transferred using SSL.
Seriously — why is iDisk so bad?