As the web becomes increasingly SSL-ified, the possibilities for mixed content — serving non-HTTP resources in an HTTPS page — increases too. This fixes that.
How does it work?
wp-expurgate adds support for expurgate to WordPress. Expurgate is a script that allows you to filter non-HTTP resources (just images, for now) through an HTTPS URL. In the eyes of browsers, this scrubs them clean and stops them causing warnings.
Got an example?
Say you have a post with the following image in it:
If a user visits your page over HTTPS, their browser will give them a warning — or in the case of IE, an ugly, scary alert box — telling them that the page isn't fully secure.
wp-expurgate will correct this to the following:
…which all browsers are happy with, since it's over HTTPS.