Lighttpd [L1 ] is a BSD-licensed open source web server that is becoming fairly popular. From its web site -

Security, speed, compliance, and flexibility -- all of these describe lighttpd (pron. lighty) which is rapidly redefining efficiency of a webserver; as it is designed and optimized for high performance environments. With a small memory footprint compared to other web-servers, effective management of the cpu-load, and advanced feature set (FastCGI, SCGI, Auth, Output-Compression, URL-Rewriting and many more) lighttpd is the perfect solution for every server that is suffering load problems.

Although claimed to be multi-platform, it is really geared towards Unix/Linux and requires Cygwin to run on Windows.