Web Caching and Web Content Filtering
Squid Web Cache
Squid Proxy Cache - An open-source, high-performance proxy caching server for web clients, supporting FTP, gopher, and HTTP data objects. Unlike traditional caching software, Squid handles all requests in a single, non-blocking, I/O-driven process. Squid is highly configurable, and is very stable under heavy loads.
Dan's Guardian is an award winning open source web content filter which currently runs on Linux, FreeBSD, OpenBSD, NetBSD, Mac OS X, HP-UX, and Solaris. It filters the actual content of pages based on many methods including phrase matching, PICS filtering and URL filtering. It does not purely filter based on a banned list of sites like lesser totally commercial filters.
Dan's Guardian is designed to be completely flexible and allows you to tailor the filtering to your exact needs. It can be as draconian or as unobtrusive as you want. The default settings are geared towards what a primary school might want but Dan's Guardian puts you in control of what you want to block or allow.