Squid is an open-source HTTP caching proxy server.
Squid is one of the best known open source HTTP proxy servers. Its features include caching of HTTP objects, URL rewriting (via helper apps), authentication, and SSL. It came out of an NSF-funded project in the early 1990s researching caching technologies.
Other similar software packages include NGINX, Apache's mod_proxy, Apache Traffic Server, and Varnish.
Squid's homepage: http://www.squid-cache.org