There is no doubt about the popularity of Apache among all available webservers, but there is one webserver with excellent performance which started to attract very important actor in the market : Lighttpd.
Lighttpd key features are security, speed, compliance, and flexibility. It have been optimized for high performance environments, with small memory footprint compared to other web-servers, effective management of the cpu-load, and advanced feature set (FastCGI, CGI, Auth, Output-Compression, URL-Rewriting and many more).
It is currently serving many known websites such YouTube, wikipedia and meebo. I think it's an excellent alternative that need to look inside it. Maybe more downloads should be available for different platforms to make adoption more easier, and also help developers to give it a try and possibly use it on their projects.
Released open source under the revised BSD license.
For more information and download http://www.lighttpd.net/