Having trouble scaling your website? Is that new marketing campaign killing your performance? Are you using a frameork or a CMS that causes too much overhead? Give Varnish a try !

Varnish is a reverse proxy that serves cached pages to your visitors instead of hitting the backend servers all the time. It's an insanely powerful tool that will rescue your site from getting crushed and is considered to be an industry standard.

Cache is stored in RAM and there's a domain specific language that allows you to hook into your caching logic. There are tools and rules and it's essentially your primary tool for when you're site is "at load".

Did I mention it works great with Drupal?