Skizzle is a Gmail add-on that helps users share end-to-end encrypted files over email. It extremely easy to use and works right alongside your regular Gmail workflow.
It uses blockchain to manage your keys and file access permissions so that users do not have to deal with them. This ensures that no one except the sender and recipient has access to files and keys, not even Skizzle.
The receiver need not be an existing user - you just need their email ID.
You can revoke access to files for a specific user.
It is intuitive and comes with a generous free plan.
Skizzle is also available as an SDK for custom application development.