nginx [engine x] is a HTTP and reverse proxy server, as well as a mail proxy server written by Igor Sysoev. It has been running for more than five years on many heavily loaded Russian sites including Rambler (RamblerMedia.com). According to Netcraft nginx erved or proxied 4.24% busiest sites in January 2010. Here are some of success stories: FastMail.FM, WordPress.com.
I have tried it and it is a must!