Since this site is all about sharing Linux based applications, I would
like to bring to your attention to an excellent Linux based content
filtering Internet proxy software - SafeSquid®.

SafeSquid®, arguably, is the world's most feature-rich proxy server.
Following, in brief, are it's features -

- Browser based GUI Interface
- Cluster friendly
- A multi-tier, md5-encrypted, totally manageable cache
- Supports ICP & CARP
- Request forwarding
- Transparent Proxying
- Reverse Proxying - Secure Web Servers
- Provides simultaneous virus scanning at the gateway using any number
of Antivirus softwares of your choice
- Generate virus signatures on-the-fly for undetected viruses while you
wait for updates
- Ready-to-use connectivity to ICAP Servers, the latest in Antivirus
Technology
- Built-in connectivity to PAM - Authenticate users from ADS, POP3,
Radius, MySQL Database, Flat File Database, etc.
- Bandwidth Management based on Users, Time, Web Sites or a combination
of all
- Block web-sites using URL Blacklist, DNS Blacklist, MySQL Database,
Category DNS and the database-independent - Weighed Keyword Scoring
Method
- Filter unwanted contents like ActiveX Controls and Java Scripts from
unknown sites (Ads, Banners, Pop-ups, spy wares, etc.)
- Prevent access / downloads based on MiMe type
- Prevent users from logging into unwanted sites - Cookie Filter
- Increase enterprise privacy - Header Filter
- Prevent access to applications like chat & IM - User Agent based
access control
- Generates log reports in Firewall and Squid format compatible with
many open source and paid Log Analysers like Sarg, Calamaris, Squint,
Webalizer, Sawmill, SquidTaild, etc.

A FREE version with limited features for non-profit / non-commercial
organisations, is available for download at
http://downloads.safesquid.net
You need to register on the SafeSquid® Forum (free) at
http://www.safesquid.com/html
You can use the forum's username and password to access the download
site.
Fully functional trial versions, with limited time period, are also
available.