Squid is one of the most useful and powerful web proxy and caching systems available. It's free and open source, which is why it can be included with IPCop. Squid itself has quite complicated configuration files and performs a variety of proxying and caching functions. As we have come to expect from IPCop, it abstracts this complication nicely and let's us configure Squid with some ease.
Squid was born as a fork of Harvest Cached, which was a proxy/caching project and released its first version in 1994, therefore Squid has had a development timeline spanning over 10 years. This has led to a quite stable and full-featured proxying and caching application. The original Harvest Cached project is no longer under development.