The Apache HTTP Server Project is an effort to develop and maintain an open-source HTTP server for modern operating systems including UNIX and Windows NT. The goal of this project is to provide a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards.
Apache's Tomcat Server, the official reference implementation for how a servlet container should support servlets. Written entirely in Java, and freely available under an open source license. All the source code is available and anyone can help with its development. This server can operate standalone or as an add-on providing Apache or other servers with servlet support. It can even be used as an embedded container. Along with Tomcat, Apache develops the standard implementation of the javax.servlet and javax.servlet.http packages. At the time of this writing servlets are the only java.* or javax.* packages officially maintained as open source.[3] See http://jakarta.apache.org.