nginx

nginx [engine x] is an HTTP and reverse proxy server, as well as a mail proxy server, written by Igor Sysoev. For a long time, it has been running on many heavily loaded Russian sites including Yandex, Mail.Ru, VKontakte, and Rambler. According to Netcraft nginx served or proxied 10.09% busiest sites in April 2012. Here are some of the success stories: FastMail.FM, Wordpress.com.

The sources are distributed under the 2-clause BSD-like license.

Serving static and index files, and autoindexing; open file descriptor cache;
Accelerated reverse proxying with caching; simple load balancing and fault tolerance;
Accelerated support with caching of FastCGI, uwsgi, SCGI, and memcached servers; simple load balancing and fault tolerance;
Modular architecture. Filters include gzipping, byte ranges, chunked responses, XSLT, SSI, and image transformation filter. Multiple SSI inclusions within a single page can be processed in parallel if they are handled by proxied or FastCGI servers;
SSL and TLS SNI support.

Name-based and IP-based virtual servers;
Keep-alive and pipelined connections support;
Flexible configuration;
Reconfiguration and upgrade of an executable without interruption of the client servicing;
Access log formats, buffered log writing, and fast log rotation;
3xx-5xx error codes redirection;
The rewrite module: URI changing using regular expressions;
Executing different functions depending on the client address;
Access control based on client IP address and HTTP Basic authentication;
Validation of HTTP referer;
The PUT, DELETE, MKCOL, COPY, and MOVE methods;
FLV and MP4 streaming;
Response rate limiting;
Limiting the number of simultaneous connections or requests coming from one address;
Embedded Perl.

User redirection to IMAP/POP3 backend using an external HTTP authentication server;
User authentication using an external HTTP authentication server and connection redirection to an internal SMTP backend;
Authentication methods:
- POP3: USER/PASS, APOP, AUTH LOGIN/PLAIN/CRAM-MD5;
- IMAP: LOGIN, AUTH LOGIN/PLAIN/CRAM-MD5;
- SMTP: AUTH LOGIN/PLAIN/CRAM-MD5;
SSL support;
STARTTLS and STLS support.

One master and several worker processes; worker processes run under an unprivileged user;
The notification methods: kqueue (FreeBSD 4.1+), epoll (Linux 2.6+), rt signals (Linux 2.2.19+), /dev/poll (Solaris 7 11/99+), event ports (Solaris 10), select, and poll;
The support of the various kqueue features including EV_CLEAR, EV_DISABLE (to temporarily disable events), NOTE_LOWAT, EV_EOF, number of available data, error codes;
sendfile (FreeBSD 3.1+, Linux 2.2+, Mac OS X 10.5+), sendfile64 (Linux 2.4.21+), and sendfilev (Solaris 8 7/01+) support;
File AIO (FreeBSD 4.3+, Linux 2.6.22+);
DIRECTIO (FreeBSD 4.4+, Linux 2.4+, Solaris 2.6+, Mac OS X);
Accept-filters (FreeBSD 4.1+) and TCP_DEFER_ACCEPT (Linux 2.4+) support;
10,000 inactive HTTP keep-alive connections take about 2.5M memory;
Data copy operations are kept to a minimum.