Varnish is a very popular software package that can dramatically accelerate the work of serving HTTP pages. Varnish caches fully-rendered responses to HTTP requests and serves them without the delay of building content from scratch. Because it's so much more efficient than building a page with Apache and Drupal, Lullabot regularly deploys Varnish when a site needs to handle high levels of anonymous traffic. In many situations, installing Varnish on the same machine as MySQL and Apache can help squeeze more performance out of a single box. However in most of our deployments, we're working with "high availability" setups