NGINX(Engine-X)

According to wiki, nginx is an open source Web server and a reverse proxy server for HTTP, SMTP, POP3 and IMAP protocols, with a strong focus on high concurrency, performance and low memory usage.

From Nginx website, it give more detail features:
NGINX is a high performance web server, caching proxy and a Layer 7 load balancing solution. Millions of web sites on the Internet benefit from using NGINX because of its extreme performance, scalability, reliability, flexibility and security.

• HTTP and HTTPS (TLS/SSL/SNI)
• Ultra-fast web server for static and dynamic content
• High concurrency HTTP proxy with load balancing and caching
• FastCGI, WSGI, SCGI, Memcached/NoSQL
• Connection and request throttling
• Comprehensive access control
• Compression and content filters
• Non-stop live binary upgrades and configuration
• Custom logging
• Embedded scripting
• Virtual hosts
• HTTP media streaming
• Mail proxy for SMTP/IMAP/POP3 with TLS
• Simple, logical, scalable configuration
• Runs on Linux, FreeBSD, Mac OS X, Solaris, AIX and Windows