The official reference implementation for Java Servlet 2.2 and JavaServer Pages 1.1, two complementary technologies developed under the Java Community Process.
Tomcat is the servlet container that is used in the implementation for Java Servlet and JavaServer Pages technologies, and will be used in the construction Of the Online Commons system. More information can be found in the Tomcat section of the Apache website. Tomcat can be retrieved from their download page.
2.4 2.0 by Apache Jakarta. Quite slow, but free, Very popular, especially on Linux. Last revised on 2006-02-15.