There only seem to be two types of people in the world when it comes to Java, The Lovers and The Haters, but there is no denying that Java is here to stay. We've got a toolbox of 20+ tools and tutorials to help out Java coders of all skill levels.
Eclipse.org - An open source development platform for Java based applications, as well as other languages.
Emma - A Sourceforge project that provides you with a free code coverage tool.
Groovy - Created to run on the Java Virtual Machine but adds in features inspired by Python, Ruby on Rails and Smalltalk.
Hibernate.org - An open source persistent classes development tool; due to licensing, it can be included in to other open source projects.
Idevelopment Java Examples - A large collection of examples of Java programming along with documentation.
JAD - A non-commercial use Java decompiler.
Java.net - A site for Java programmers and enthusiasts to gather, chat, and collaborate on projects together.
Java PathFinder - A verifier for executable Java bytecode hosted by Sourceforge.
Java-Source.net - A directory to just about every open source Java project you could dream of.
Java-Tips.org - A huge collection of hundreds of tips related to Java programming.
Java2s.com - Tons of examples of Java code.
JavaBlackBelt.com - Take online courses, followed up by tests that let you climb the colored belt tree (like you do in martial arts) until you become a Java Master.
Javalobby.org - Articles, forums, and more to learn all sorts of tricks for working with Java.
JavaPractices.com - Tons of examples of different functions with snippets of source code provided.
JavaRanch.com - A user community for Java programmers with a ranch-flavor. Forums, tools, code snippets and more.
JDocs.com - A huge resource of libraries, packages, and classes.
JExamples.com - A large collection of examples of Java from various open source projects that you can study.
JGoodies.com - Tools focusing on the look of Java in an attempt to make it look more elegant.
JODE - Free Sourceforge project that contains a decompiler and optimizer for Java code.
JUnit.org - An open source testing platform for Java that works towards understanding your intentions.
PHP/Java Bridge - A Sourceforge project that aids in bridging the gap between PHP and Java to make them work together.
PMD - A Sourceforge project that scans Java apps and looks for bad code.
SpringFramework.org - A Java/JEE application framework with a large support system and training sessions all over the world.
The Grinder - Another Sourceforge project that is a Java load testing framework.
TheServerSide.com - A Java developer community with a focus on enterprise-class projects. (via)