nginx - Basically, it's saved a server.
Quote of the day about nginx web server :
I've moved two phpAdsNew ad servers, a fairly-busy Wordpress blog, the above-mentioned Drupal site and my own Drupal site from Apache2 into Nginx in the last week. Processor use has increased on the server, but critically the ongoing web+database memory use has come down by over 300MB. This means that the box hasn't gone into swap for a week (normally it was paging out every day), server response times are up and (because I'm also using APC to store Drupal's source files in memory) page download speeds are also up considerably.
Basically, it's saved a server.