Yep. Squid is a plain WWW-proxy with no server capabilities, it provides ways to hook several caches to a cache-network, though. For more information, see http://squid.nlanr.net/Squid/.
Also, the CERN and Roxen (ex-Spinner) are known to have Proxy/Cache-capabilities, as well as the latest versions of Apache seem to have. See previous question for more details.