Jakarta Tomcat is a web server written in 100% Pure Java.

Tomcat 5.x is the current release of Tomcat, 
and builds upon the Tomcat 3.3 and Tomcat 4.1 codebases. 
The 5.x releases implement the Servlet 2.4 and JSP 2.0 specifications.

Apache Tomcat 5.5 is the current focus of development.
While it supports the same Servlet and JSP Specification versions
as Apache Tomcat 5.0.x, there are significant changes in many areas
under the hood, resulting in improved performance, stability,
and total cost of ownership.

WWW: http://jakarta.apache.org/tomcat/