nginx

nginx.gif nginx (pronounced engine x) is a http server and mail proxy server that is open sourced and extremely scalable. It’s used on many Russian sites, but one of the major success has been Fastmail.fm. nginx is written in such a way where it doesn’t use the usual one connection, one process method as most linux type server daemons run. Instead, it uses a polling feature, that allows the overhead to be extremely low. Thus, you’re able to maximize many more connections without having to use many resources (10,000 inactive HTTP keep-alive connections take about 2.5M memory).
So if you’re looking for a highly scalable web server or mail proxy server, nginx could be your key to decreasing your load and increasing your performance.