I'm sure there were companies that were doing the same thing as Dropbox at the time that Dropbox was released. To name one example - Microsoft's SkyDrive (apparently called Windows Live Folders at the time), released either around the same time or prior to DropBox, depending on your definition. It's been a while since I've used it, but I remember the functionality being roughly equivalent between the two products (in that they fulfill the base use case of silent synching between two computers)

That isn't to say Dropbox didn't blow them out of the water in regards to execution, but it wasn't an idea that was completely without precedent. Ideas rarely are, even if they seem like that in retrospect due to one company out executing everyone to an insane degree.