GoAhead WebServer
aprelium - Abyss Web Server offers a free personal edition, SourceForge.net - ABYSS Web Server contains the last open source version from 2000.

Pi3Web
Apache HTTP Server (License: Apache License Version 2.0)
nginx is a HTTP server and mail proxy server written by Igor Sysoev. (Windows versie.) (License: 2-clause BSD-like license)
Cherokee (License: GNU GPL)
thttpd - tiny/turbo/throttling HTTP server (License: BSD/MIT-style)
sthttpd a fork of Jef Poskanzer's popular thttpd server (License: BSD/MIT-style)
lighttpd - designed and optimized for high performance environments. (Windows version.) (License: Revised BSD)
Yaws is a HTTP high perfomance 1.1 webserver particularly well suited for dynamic-content web applications. It is entirely written in Erlang. (License: BSD/MIT-style)
nostromo
webfs a simple http server for mostly static content. (License: GNU GPL)
althttpd see Re: [fossil-users] Fossil behind proxy and CVSTrac Server Configuration for more information.
Roxen WebServer is a full-featured open-source web server distributed under the GPL license. It runs on a number of different operating systems including Windows, Linux, Solaris and Mac OS X.
KLone is a fully-featured, multiplatform, web application development framework, targeted especially for embedded systems and appliances. (License: Simplified BSD license)
gatling - a high performance web server (License: GNU GPL)
fnord - yet another small httpd (License: GNU GPL)
Xitami
Monkey Web Server (License: Apache License Version 2.0)
unix4lyfe.org » darkhttpd (License: BSD)
ACME Laboratories - mini_httpd - small HTTP server (License: BSD-style)
ACME Laboratories - micro_httpd - really small HTTP server (License: BSD-style)