A term for any cgi script on a web page which reads in another web page and gives it back to you altered. Publicly available examples include: Mediators will generally take the page to be mediated as their CGI env "argument", and then adjust all links within the mediated page to filtered through the mediator; however this is not a definite rule, and Babelfish, for example, stops you at an intermediary page with banner ads every time you click a link into babelfish.

The core behind Shoudoka, the Malkovich Mediator, and crit.org is open-source and publicly available at crit.org. You can easily create your own arbitrary mediator with very little effort by using the code publicly available there as a base.

A more complete list is at http://www.lfw.org/ping/mediator.html
The first five examples above have been written up. The rest have not. Msg me if you write any of these up and i will add a link.