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Nginx is a high-performance web and proxy server. It has a lot of features

As an Apache replacement that gracefully handles many concurrent connections: Nginx is especially popular among web hosting providers. 50,000 simultaneous connections have been reported, thanks to Nginx’s use of epoll and kqueue.

As a load-balancing proxy server: Nginx is a popular front-end to Rails and PHP applications, either via FastCGI or HTTP. Written in C, it consumes about a quarter of the CPU that Perlbal uses.

As a mail proxy server: more of a niche application, but fastmail.fm reports great success.

As a server with a simple installation process, a clean configuration file, and few bugs: Nginx is easy to get running, and it almost never needs restarting. You can even upgrade the binary with zero downtime.