Varnish, the web application accelerator:
Varnish development was initiated as a project within VG as a direct response to increasing demand hitting our servers hard. Existing caching systems were simply not fast or flexible enough to deal with VGs needs; and so Varnish was born with it’s first official release in 2006.
Varnish is now developed and maintained under a separate company (https://www.varnish-cache.org). It is open source (two-clause BSD license) and used among many large web applications including VG.