A Simple Multithreaded Web Server: A tutorial article from java.sun.com with the source code for a working minimalist webserver.
Bajie Java Http Web Server: Includes installation guide, FAQ, bug list, and source.
Dwarf HTTP Server: Fully-featured web server with Java Servlet API 2.2 and Java Server Pages 1.1 implementation. [Shareware]
Jetty HTTP Server: A HTTP Server and Servlet container written in 100% Java. [Open source, Artistic-like License]
Jigsaw: The official W3C Java-based Web server. (W3C is the WWW standards body.) With a modular architecture and full HTTP/1.1 compliance, the Jigsaw server is an experimental platform for W3C and the Internet community.[Open Source]
Tynamo Web Server: A fast, conditionally compliant HTTP/1.1 server and Servlet 2.2 container specifically designed for embedded Java devices, including TINI, JStik, SNAP, and EJC.
Vq Server: Java server for Windows 9x/NT and Mac. [Freeware, requires registration]
WebServer: A web server written in Java. Supports CGI, PHP, and Perl. Requires Unix-like OS. [Open source, GPL]