The Apache Software Foundation's Jakarta project, which provides an umbrella for a number of Java-related projects, is at http://jakarta.apache.org/. Tomcat, part of the Jakarta project, is working to write an open-source reference implementation of Sun's servlet and JSP specifications. The Jakarta web site provides links to the latest source, binary and development versions of Tomcat.
An excellent resource on the subject of JSPs is the recently published O'Reilly book, JavaServer Pages, written by Hans Bergsten. In my opinion, the book leans too heavily in the direction of custom actions, but if you are looking to provide this sort of functionality, or simply learn about ways in which to write and use JSPs, this book is a great place to start.
There are also a number of on-line tutorials about writing custom tags for JSPs. The Jakarta-Tomcat site has one such tutorial, found at http://jakarta.apache.org/taglibs/tutorial.html.
Another tutorial on the subject is available from Sun itself, at http://java.sun.com/products/jsp/tutorial/TagLibrariesTOC.html.