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

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/

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

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

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/

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

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

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/

Internet Data Filtering Talk
Presentation by Christopher Browne to the North Texas Linux Users Group (NTLUG) in October 1997.
http://cbbrowne.com/info/ifilter.html

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/

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

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/

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

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

Norton Internet Security
Commercial software for Windows 98/Me/2000/XP. Mainly used as a firewall and anti-virus checker, but has ad blocking and parental control capabilities.
http://www.symantec.com/home_homeoffice/products/category.jsp?pcid=is

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