Squid

[http://www.squid-cache.org/ Squid] is a high-performance proxy server that can also be used as a HTTP accelerator for the webserver. See:
 * Squid Reverse Proxy
 * Manual:Squid caching

and associated configuration variables:
 * $wgUseSquid
 * $wgSquidServers
 * $wgSquidServersNoPurge