Many open source libraries/frameworks exist for Java that can make coding certain tasks significantly easier, but finding these libraries/frameworks can be a different matter entirely! This article is devoted to compiling some of the more common useful Java libraries and some of the lesser known libraries.

Apache Commons: ..................  http://commons.apache.org/  Huge collection of useful components to help manage 'common' coding tasks. Commons.Lang and Commons.IO are extremely. 

Google Collections/Guava: .....  http://code.google.com/p/guava-libraries/  Google's collections API and many other useful core libraries that help to extend the core Java libraries.

JUnit:  ........................................... http://www.junit.org/  A good xUnit test framework for Java.

Hibernate: ................................... http://www.hibernate.org/  Helps facilitate the storage and retrieval Java domain objects through Object/Relational mappings.

Joda-Time:  ................................ http://joda-time.sourceforge.net/   Makes dealing with date and time calculations and objects significantly easier.

Spring: ........................................ http://www.springframework.org/  Provides extra functionality for everyday applications but mainly used in the creation and management of web applications. 

Lucene: ...................................... http://lucene.apache.org/java/docs/   "...is a high-performance, full-featured text search engine library written entirely in Java."

JFreeCharts: ............................. http://www.jfree.org/jfreechart/  Considered one of the top charting/graph libraries for Java.  

iText: ........................................... http://www.lowagie.com/iText/  PDF creations and manipulation.

Log4J: ......................................... http://logging.apache.org/log4j/1.2/index.html   A good library devoted to logging.

XStream:..................................... http://xstream.codehaus.org/  "Out of the box POJO serialization to XML and back again" - Pyrolistical

SWT:............................................. http://www.eclipse.org/swt/  Libraries for creating extensive UIs in Java.

SwingLabs:  ............................... https://swingx.dev.java.net/  A library for extending the functionality provided by Java's Swing component.

Jetty: ........................................... http://www.mortbay.org/jetty/  "Easy-to-use, full-featured, embeddable web server and webapp container" - David Crow

JDOM: .......................................... http://www.jdom.org/  XML and DOM manipulation for Java.

Selenium: ................................... http://seleniumhq.org/  Write tests in JUnit style, then run automated tests against them in Firefox.

JAI: ............................................... http://java.sun.com/javase/technologies/desktop/media/jai/   "... provides a set of object-oriented interfaces that supports a simple, high-level programming model which allows images to be manipulated easily in Java applications and applets. JAI goes beyond the functionality of traditional imaging APIs to provide a high-performance, platform-independent, extensible image processing framework."

Velocity: ...................................... http://velocity.apache.org/index.html  Create text (SQL Queries, HTML, etcetera) from predefined templates.

jXLS: ............................................. http://jxls.sourceforge.net/  Create XML files from templates.

Google Web Toolkit (GWT): ..... http://code.google.com/webtoolkit/  Code in Java, then compile to JavaScript.  Useful for creating good looking AJAX web applications.

Processing: ................................ http://www.processing.org/   A powerful graphics and multimedia for Java.  Has also been ported over as a JavaScript version.

DOM4J: ......................................... http://www.dom4j.org/  XML/Xpath/XSLT processing for Java.