Apache Tomcat - An Open Source JSP and Servlet Container from the Apache Foundation. - http://tomcat.apache.org/
PolyJsp - An extensible JSP implementation designed to support multiple scripting languages and multiple JSP versions. [Open Source] - http://www.plenix.org/older/polyjsp/
Bea WebLogic Server - J2EE Certified Application Server and JSP Engine - http://www.bea.com/framework.jsp?CNT=index.htm&FP=/content/products/server
GNUJSP - Free implementation of Sun's Java Server Pages. [Open Source, GPL] - http://www.klomp.org/gnujsp/
Resin - Java based XSL focused on dynamic server pages. This Java servlet supports JavaScript and JavaServer Pages (JSP). It also compiles JavaScript templates into Java byte code. [Commercial] - http://www.caucho.com/
Jigsaw Java Server - W3C's open source web server and servlet engine. - http://www.w3.org/Jigsaw/