Content filtering is the technique whereby content is blocked or allowed based on analysis of its content, rather than its source or other criteria. It is most widely used on the internet to filter email and web access.
See also:
Privoxy
privoxy.org
A web proxy with advanced filtering capabilities for protecting privacy, modifying web page content, managing cookies, controlling access, and removing ads, banners, pop-ups and other Internet junk. Based on Internet Junkbuster.
Proxomitron, The
proxomitron.info
General proxy with text-matching rules for transforming and filtering pages and HTTP headers. Includes filters for animated GIFs, pop-up windows, ad banners, and dynamic HTML.
Muffin
muffin.doit.org
Open-source cross-platform filtering Web proxy server written in Java. It has a public API to create additional filters.