A GPLed filtering web proxy, designed for schools and colleges but suitable for many other environments.
CensorNet is based on the Debian distribution and is supplied as a ISO image. It uses GPL projects like Dan's Guardian, Squid and Samba to do the donkey work, but ties it all up in an excellent web interface. The project includes close integration with a Windows domain (entirely optional but a wise choice given the target audience) and offers excellent reporting features.
The CensorNet distribution is GPLed, but commercial support and development is available. An image filtering module is also available, but this is quite pricey. Adelix, the CensorNet developers, sell "out-of-the-box" solutions, as well as on-site support and custom development. One important feature is the BLUD, which updates the system blacklist. New blacklists are published everyday, so systems can automatically stay up to date. This service costs £100 a year, which is a great price to show support for the company in their efforts developing GPLed software. (The updates are from Dan's Guardian, but tweaked for CensorNet users.) Users do, of course, retain control over the exact list of filtered sites, and the restrictions that are put in place. All the filtering methods can be set to a timetable, allowing internet access to be automatically switched off after school, for example.
CensorNet requires minimal hardware. TonyWhitmore runs CensorNet on a 2.4Ghz P4 system, with 1GB RAM and an 80GB IDE HDD. However, the hardware was chosen to leave an upgrade path available. It currently proxies over 450 machines and shows a quite heavy load at peak time. However performance is still acceptable. Like IPCop or SmoothWall, CensorNet will take over the hard disc(s) in the host system, but the only other requirement is to have two supported NICs installed.
New versions of CensorNet feature improved performance from the database back-end and support for SCSI storage devices. The homepage features clear documentation, and a moderately lively user forum exists.