Varnish, or strictly Varnish Cache, is a mature web server http caching reverse proxy that speeds up server replies.
Varnish Cache is a free open-source web server accelerator, using memory to hold replies, and relying on the OS to page out lesser-used information. It dates from 2006 and is in use by several major web-sites.
It has its own configuration language, VCL, and can be extended with VMOD modules, many of which are already available.
The commercial version has products based on Varnish Cache, such as Varnish Plus and Varnish API Engine.