There are numerous automated download managers for Windows supporting major file-hosters like Rapidshare, Megaupload and Mediafire, but if you are looking for something which gets the job done on other platforms like Linux, open-source utility "Tucan Manager" would be the answer, developed in Phython the cross-platform utility lets users upload and download files on Rapidshare, Megaupload, Gigasize and Mediafire supporting both free and premium account functionality.
The fast and simple to use utility supports plugins making it easily extensible with other services, the application is in ALPHA stages and definitely posses a lot of potential considering its open-source multi-platform nature.