Is there an open source implementation of servlets?
Yes, Tomcat is an open source implementation of JSP and Java Servlets by the apache group (i.e. it provides a Servlet 2.2/JSP 1.1 compliant container). It runs as a stand alone web server or can be integrated with apache with mod_jk. This server is based on technology originally developed by Sun. It runs on Unix/Linux and Windows; it took me about an hour to install and get it working on WinME to the point where I could successfully modify the JSP examples included with the distribution. It took me just a few minutes more to build and test a new servlet. Really, very amazing stuff. jetty is 100% Java HTTP Server and Servlet Container.