Apache Derby, an Apache DB subproject, is an open source relational database implemented entirely in Java and available under the Apache License, Version 2.0. Some key advantages include:
Derby has a small footprint -- about 2 megabytes for the base engine and embedded JDBC driver.
Derby is based on the Java, JDBC, and SQL standards.
Derby provides an embedded JDBC driver that lets you embed Derby in any Java-based solution.
Derby also supports the more familiar client/server mode with the Derby Network Client JDBC driver and Derby Network Server.
Derby is easy to install, deploy, and use.

The goal of the Geronimo project is to produce a server runtime framework that pulls together the best Open Source alternatives to create runtimes that meet the needs of developers and system administrators. Our most popular distribution is a fully certified J2EE 1.4 application server runtime.

Gump is Apache's continuous integration tool. It is written in python and fully supports Apache Ant, Apache Maven (1.x and 2.x) and other build tools. Gump is unique in that it builds and compiles software against the latest development versions of those projects. This allows gump to detect potentially incompatible changes to that software just a few hours after those changes are checked into the version control system. Notifications are sent to the project team as soon as such a change is detected, referencing more detailed reports available online.

Apache Jackrabbit is a fully conforming implementation of the Content Repository for Java Technology API (JCR). A content repository is a hierarchical content store with support for structured and unstructured content, full text search, versioning, transactions, observation, and more. Typical applications that use content repositories include content management, document management, and records management systems.

Apache JMeter is a 100% pure Java desktop application designed to load test functional behavior and measure performance. It was originally designed for testing Web Applications but has since expanded to other test functions. Apache JMeter may be used to test performance both on static and dynamic resources (files, Servlets, Perl scripts, Java Objects, Data Bases and Queries, FTP Servers and more). It can be used to simulate a heavy load on a server, network or object to test its strength or to analyze overall performance under different load types. You can use it to make a graphical analysis of performance or to test your server/script/object behavior under heavy concurrent load.

The POI project consists of APIs for manipulating various file formats based upon Microsoft's OLE 2 Compound Document format, and Office OpenXML format, using pure Java. In short, you can read and write MS Excel files using Java. In addition, you can read and write MS Word and MS PowerPoint files using Java.

Apache ServiceMix is an open source ESB (Enterprise Service Bus) that combines the functionality of a Service Oriented Architecture (SOA) and an Event Driven Architecture (EDA) to create an agile, enterprise ESB.

Apache Struts is a free open-source framework for creating Java web applications. Web applications differ from conventional websites in that web applications can create a dynamic response. Many websites deliver only static pages. A web application can interact with databases and business logic engines to customize a response. Web applications based on JavaServer Pages sometimes commingle database code, page design code, and control flow code. In practice, we find that unless these concerns are separated, larger applications become difficult to maintain. One way to separate concerns in a software application is to use a Model-View-Controller (MVC) architecture. The Model represents the business or database code, the View represents the page design code, and the Controller represents the navigational code. The Struts framework is designed to help developers create web applications that utilize a MVC architecture.

Apache Velocity is a template processor that can be used for more flexible and complex Java source-code generation.

APIviz is a JavaDoc doclet which extends the Java standard doclet. It generates comprehensive UML-like class and package diagrams for quick understanding of the overall API structure.