SQUID proxy running on a linux box. preferably debian.
Squid is...
* a full-featured Web proxy cache
* designed to run on Unix systems
* free, open-source software
* the result of many contributions by unpaid (and paid) volunteers
Squid supports...
* proxying and caching of HTTP, FTP, and other URLs
* proxying for SSL
* cache hierarchies
* ICP, HTCP, CARP, Cache Digests
* transparent caching
* WCCP (Squid v2.3 and above)
* extensive access controls
* HTTP server acceleration
* SNMP
* caching of DNS lookups
http://www.squid-cache.org/