Anyone using SpiderOak? They're the only other versioning diff-based backup service I know of that supports Linux (with a free tier; there's also Tarsnap). They also claim 'zero-knowledge' encryption. Anyone have any opinions?
There's also ownCloud ( http://owncloud.org/index.php/Main_Page ), if you're into that whole home server thing. It definitely supports Linux.
(WebDAV isn't "support," when you look at the Dropbox/SpiderOak feature set.)