Lighttpd is a very small and very efficient web server. It is a simple and powerful alternative to IIS and Apache web servers. You can see some benchmarks on Lighttpd benchmark page. There are also links to other tests on this page so developers are pretty honest about Lighttpd performance.
Lighttpd is very easy to install and easy to configure. To install Lighttpd on Windows run the setup file and install it into a folder. Then edit the configuration file to suit your needs. After all changes have been applied you can execute start-lighttpd.bat and the webserver will come online.
You can play around with settings through .conf file.
The fun part is that Lighttpd is available for a large number of various operating systems including Linksys routers. So if you have a linksys router you might as well turn it into a webserver showing stats and doing other interesting things.
Lighttpd is pretty stable and easy to use. I guess it is a simple tool for all users who need a simple webserver for small projects, situations when IIS is not available, or just high performance servers.
Lighttpd can be installed as a package for Cygwin or Windows Install package(does not require Cygwin). For some reason the site with Windows install package is not available fat the moment so I ve mirrored the package here: Lighttpd for Windows 1.4.11 .
P.S. If you decide to use Lighttpd in production make sure you follow security advisories for it.