Ciphire Mail

This looks interesting. A pretty painless way to digitally sign/encrypt email, regardless of the email client you use (although it doesn't work with any of the web-based email systems). They say that it will always be free for private users, non-profit organizations, educational institutions, and the press.

A paid-for enterprise variant is on its way.

See https://www.ciphire.com/ for the full run-down.

[edit] - It's currently in beta, and they are looking for beta-testers at the moment, according to the site.