You may have noticed that if you try to switch the front-end of your WordPress site over to using HTTPS instead of HTTP, legacy content that contains image URIs and links to internal posts and pages on your site will not change automatically (as they would when you use wp-admin).
HTTPS All the Things is a configuration-less plugin that aims to help you out by filtering those URIs on output and altering them to use HTTPS. (It will also work in reverse if needed.)
What it does is take the URI scheme that is currently configured in your site’s Site Address value in Settings > General and ensures that resource URIs conform to that scheme when it can. Some themes and plugins may generate URIs using methods that will bypass this plugin; in those cases you may occasionally need a compatibility shim to ensure compatibility.
If you need to test your site for mixed content problems, you may want to take a look at this analysis plugin from Zack Tollman.
The plugin is forked from GPL2 code that was originally written by John Blackbourn and the team at Code for the People. It is likewise released under GPL2 and the source is available on GitHub. Issues and pull requests are welcome.