Dropbox
Regarding Dropbox and folders - sort of a yes-and-no answer - whatever folder structure you set up on your pc will show up the same way on the iPad, but you can't 'transfer' the folders to your photo library.
Another good use of Dropbox is for transferring pdf files, however I usually use Good Reader for that as I can then access them offline, where as Dropbox requires a wifi connection to access the cloud.