Content Filtering

Muffin - Open-source cross-platform filtering Web proxy server written in Java. It has a public API to create additional filters.
http://muffin.doit.org/

Mutating Web proxy (RabbIT) - A Java application which will cache, compress images and remove unwanted content in order to speed up the browsing process.
http://rabbit-proxy.sourceforge.net/

FilterProxy - An HTML-rewriting proxy with Web-forms based configuration and modules for XSLT, GIF deanimation, and compression. [Perl]
http://filterproxy.sourceforge.net/

Using a Hosts File To Make The Internet Not Suck (as much) - Basic introduction to hosts file usage.
http://someonewhocares.org/hosts/

WebSense - Internet access management system allows an organization to monitor, report and manage internal Internet use.
http://www.websense.com/

LANguard - Keyword-based content filtering and virus scanning for Microsoft ISA Server.
http://www.gfi.com/dsec/

SquidGuard - A combined filter, redirector and access controller plugin for Squid.
http://www.squidguard.org/

Norton Internet Security - Firewall, anti-malware, anti-spam and phishing protection software which includes parental control add-on for filtering unsuitable websites. [Windows]
http://www.symantec.com/norton/internet-security

Application-Specific Proxy Servers as HTTP Stream Transducers - Paper from W3C discussing research on an application specific Web proxy.
http://www.w3.org/Conferences/WWW4/Papers/56/

Proxomitron, The - 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.
http://www.proxomitron.info/

WebFilter - Extension to Cern's httpd web server to filter out annoying parts of web pages.
http://www-math.uni-paderborn.de/~axel/NoShit/

PureSight - PureSight is a password protected, easy to use program, that runs silently on the computer and controls all of the communication to and from the PC.
http://www.puresight.net/

X-Stop Internet Access Management Solutions - X-Stop allows homes and schools to provide safe Internet access and corporations to manage their employee's Internet usage. It facilitates an organization's goal of maintaining employee and network productivity while reducing the risk of legal liability.
http://www.xstop.com

Privoxy - 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.
http://www.privoxy.org/

Configuring Your Mac to Work With Filter Proxy Settings - Instructions and links to relevant tools.
http://macfilter.freeyellow.com/

8e6 Technologies - Maker of stand-alone external Internet content filtering appliances. [Windows]
http://www.8e6.com/