Java Open-Source Tools

Java Unit Testing – Junit

Swing GUI - jfcUnit

Html - HtmlUnit, WebTest

Servlets/EJB - Cactus

ANT - ANT stands for Another Neat Tool, and it lives up to its name. ANT takes your source code and builds it into an application—sort of.

CruiseControl - CruiseControl is a scheduling and reporting tool that uses ANT to build your system, and then runs all of its tests unit, integration, and acceptance (if they all exist, which they should). It then reports on the results in a variety of ways, including uploading HTML reports to a web server.

Castor - Castor is a relational-hierarchical-object mapping tool. It supports Java-to-XML , Java-to-SQL binding and lot more.

MySql - World's most popular OpenSource Databases. It's a general purpose database, and it comes with good JDBC drivers. (ConnectorJ is the official one)

Hibernate - OR mapping tool

Axis - Axis is a SOAP server from Jakarta. It implements SAAJ (SOAP with Attachments API for Java) from Sun, and SOAP 3 from W3C.

Tomcat - Servlet Engine
Catalina - the servlet container itself
Jasper - the JSP translator (it translates JSP source into Java source for a servlet)
Coyote - a web server

Reference:
The Best Java Open-Source Tools - http://www.informit.com/articles/article.aspx?p=29962

Java OpenSource Testing Tools - http://www.opensourcetesting.org/unit_java.php