Dropbox
Dropbox is certainly a promising technology—in many ways, what iDisk should have been—but it currently has some serious problems. It does not support Mac resource forks, extended attributes, or packages. Worse, it does not tell you that it doesn’t support these features. It just silently throws away those parts of your files. Until this is fixed, it should only be used by technical users who are sure that these limitations will not cause problems for them.
Update (2010-12-17): Dropbox 1.0.
