HTTPS Everywhere is by far one of the most important browser plugins, perhaps ever. It attempts to ensure that a user is communicating securely with a remote site. It does this by comparing the URL being requested to a database of known HTTPS (read: “secure”) endpoints, and rewrites the requested URL if it knows of a suitable replacement. (It, of course, falls back to the plain ole, HTTP endpoint as necessary)
In addition, and optionally, it participates in the HTTPS Observatory, a project that collects information about SSL/TLS certificates in use in the real world, by real web sites, and studies them, alerting users of potential problems in real-time, and capturing and analyzing trends to help ensure our future communications remain secure.
I recently built a library, in Go, for the rulesets that the HTTPS Everywhere plugin utilizes to define the HTTP to HTTPS mapping. The obvious next thing to do was to wrap it up in a little proxy, that runs on localhost, such that one can safely utilize the web when a HTTPS Everywhere supporting browser isn’t in use.
Checkout heproxy on Github. There are likely a million improvements, and next steps (like incorporating some privoxy type things, or maybe just porting this into privoxy to begin with?) to be made, but have at it, at your own risk of course.