吉祥三宝

http://struts.apache.org/

http://www.springframework.org/

http://www.hibernate.org/

Apache

http://ant.apache.org/
Apache Ant is a Java-based build tool. In theory, it is kind of like Make, but without Make's wrinkles.

http://lucene.apache.org/
Apache Lucene is a high-performance, full-featured text search engine library written entirely in Java. It is a technology suitable for nearly any application that requires full-text search, especially cross-platform.

http://jakarta.apache.org/commons/index.html
The Commons is a Jakarta subproject focused on all aspects of reusable Java components.

http://jakarta.apache.org/velocity/index.html
Velocity is a Java-based template engine. It permits anyone to use a simple yet powerful template language to reference objects defined in Java code.

Sourceforge

http://junit.sourceforge.net/
JUnit is a simple framework to write repeatable tests. It is an instance of the xUnit architecture for unit testing frameworks.

http://httpunit.sourceforge.net/
HttpUnit makes you easy  to be able to bypass the browser and access your site from a program. When combined with a framework such as JUnit, it is fairly easy to write tests that very quickly verify the functioning of a web site.

http://dozer.sourceforge.net/
Dozer is a powerful, yet simple Java Bean to Java Bean mapper that recursively copies data from one object to another. Typically, these Java Beans will be of different complex types.

http://xdoclet.sourceforge.net/xdoclet/index.html
XDoclet is an open source code generation engine. It enables Attribute-Oriented Programming for java. In short, this means that you can add more significance to your code by adding meta data (attributes) to your java sources. This is done in special JavaDoc tags.

http://displaytag.sourceforge.net
The display tag library is an open source suite of custom tags that provide high-level web presentation patterns which will work in an MVC model. Actually the display tag library can just... display tables! Give it a list of objects and it will handle column display, sorting, paging, cropping, grouping, exporting, smart linking and decoration of a table in a customizable XHTML style.

http://cglib.sourceforge.net/
cglib is a powerful, high performance and quality Code Generation Library, It is used to extend JAVA classes and implements interfaces at runtime.

Others

http://www.opensymphony.com/compass/
Compass is a first class open source Java Search Engine Framework, enabling the power of Search Engine semantics to your application stack decoratively. Built on top of the amazing Lucene Search Engine, Compass integrates seamlessly to popular development frameworks like Hibernate and Spring.

http://www.opensymphony.com/quartz/
Quartz is a full-featured, open source job scheduling system that can be integrated with, or used along side virtually any J2EE or J2SE application - from the smallest stand-alone application to the largest e-commerce system.

http://groovy.codehaus.org/
Groovy is an agile dynamic language for the Java Platform with many features that are inspired by languages like Python, Ruby and Smalltalk, making them available to Java developers using a Java-like syntax.

http://www.singularsys.com/jep/
JEP is a Java library for parsing and evaluating mathematical expressions. With this package you can allow your users to enter an arbitrary formula as a string, and instantly evaluate it. JEP supports user defined variables, constants, and functions. A number of common mathematical functions and constants are included.

http://www.dom4j.org/
dom4j is an easy to use, open source library for working with XML, XPath and XSLT on the Java platform using the Java Collections Framework and with full support for DOM, SAX and JAXP.

http://www.castor.org/
Castor is an Open Source data binding framework for Java. It's the shortest path between Java objects, XML documents and relational tables. Castor provides Java-to-XML binding, Java-to-SQL persistence, and more.