I was typing another comment just as you posted this but making essentially the same point.
If you are using Linux, FreeBSD or Mac OSX you can use local open source encryption before your data touches the sync program (or even the online backup servers) with EncFS.[1] I'm using it with Dropbox and it is solid, easy to use and set up, and does efficient file-based encryption (so no re-uploading folders just because you changed a file inside).
Gnome desktops even have the Gnome EncFS Manager[2] if you want a GUI way to set it up.
Thanks for the info, but I said ubiquitous exactly because I need something that "just works" on each one of Windows, Linux, Android and iOS. And I'm not aware of any open source solution there, hence have to use Wuala.
IIRC, Boxcryptor isn't specific to iOS, I have it running on OSX at home, and I believe it also runs on Windows, though I've never had a reason to put it on my VM.
If you are using Linux, FreeBSD or Mac OSX you can use local open source encryption before your data touches the sync program (or even the online backup servers) with EncFS.[1] I'm using it with Dropbox and it is solid, easy to use and set up, and does efficient file-based encryption (so no re-uploading folders just because you changed a file inside).
Gnome desktops even have the Gnome EncFS Manager[2] if you want a GUI way to set it up.
[1]https://en.wikipedia.org/wiki/EncFS [2]https://bitbucket.org/obensonne/gnome-encfs/