Brazil
Brazil is a distributed content Web application server. The framework enables stand-alone systems to work together within the Web space, and a strong authentication architecture allows extranets to access intranets in an open environment without compromising security.

Fizmez Web Server
TheFizmez Web Server is GPL, meaning that it is free to distribute and use. It is a very simple webserver, written in Java and completely portable to any Java environment which supports sockets.

Jetty
Jetty is a 100% Java HTTP Server and Servlet Container. This means that you do not need to configure and run a separate web server (like Apache) in order to use java, servlets and JSPs to generate dynamic content. Jetty is a fully featured web server for static and dynamic content.

Jigsaw
Jigsaw is W3C's leading-edge Web server platform, providing a sample HTTP 1.1 implementation and a variety of other features on top of an advanced architecture implemented in Java.

jo!
jo! is a pure, free Java web server that implements the Servlet API 2.2, JSP 1.1 and HTTP/1.1. It features: HTTP/1.1 including byte-ranges, auto-chunking, pipelining, etc; Servlet API 2.2 (2.3 starting with jo! 1.1); JSP 1.1 (1.2 starting with jo! 1.1); Auto reload of WARs; Hot deployment of WARs.

Miniature Java Web Server
The Miniature Java Web Server includes most of functionality of real Web Servers including running servlets. The server is pretty small as in Java code as in result byte code. General  purpose of the Web server is running and debugging servlets. However, if you host not so high loaded web site, you can use the server as a regular web server with some benefits against big brothers.

NanoHTTPD
A free, simple, tiny (1 java file!), nicely embeddable HTTP server in Java.

Pygmy
Pygmy is a tiny webserver ment for embedding into applications. The core of the server around 40KB. It provides a flexible way to plug-in features, similiar to apache modules or servlets, but it's a much more lightweight API. However

Reattore
Reattore is a simple single threaded HTTP server written in Java. Unlike most Java server applications, Reattore uses the socket channel features added in Java 1.4 to serve all requests from one thread, instead of spawning each request off to a new thread. In theory this provides better performance and allows the system to degrade well under high load.

Resin
The Resin® high-performance application server features load balancing for increased reliability. Resin encourages separation of content from style with its fast XML and XSL support.