Jos Poortvliet gave an interesting presentation at 34C3 regarding NextCloud’s end-to-end encryption features.
NextCloud is a host-it-yourself cloud solution that includes file syncing, document editing, calendar, etc.
It also features an end-to-end encryption option which, as Poortvliet nicely summarizes it, protects user data from the server itself. If everything works correctly (and this feature is still under development), even if someone compromises the server on which you’re hosting NextCloud, they won’t be able to get at the data because the server doesn’t have the ability to unencrypt the data without permission of the user.
Currently in proof-of-concept phase and scheduled to be part of Nextcloud 13, it enables users to pick one or more folders on their desktop or mobile client for end-to-end encryption. Folders can be shared with other users and synced between devices but are not readable by the server.