Sun's Java home page is the authoritative page for Sun originated Java material. There is a lot here including many "add on" packages that are not included in the Java SE distribution.

Eclipse is a development environment which can be used for developing Java systems. It is being developed by a consortium of the major players in the Java (and C++) worlds. In effect it is the de facto standard IDE.

Netbeans is Sun's Java development IDE, it is competition for Eclipse.

IntelliJ IDEA is a commercial Java development IDE that a lot of people think is the very best development environment for Java – even though it costs money!

TestNG is a unit test framework for Java code – arguably the de facto standard for Java 5 development.

JUnit the original de facto standard unit test framework.

Ant the de facto standard build tool for Java programming.

Maven a build system for Java projects that many people think is far better than Ant, though some think it is truly horrible and can never replace Ant.

Subversion a tool for managing source code, rapidly replacing CVS as the standard tool for open source projects and commercial projects alike.

IBM developerWorks opensource is the site IBM maintain to host various open source projects.

ObjectSpace's Java Generic Library (JGL) – now owned by Recusion Software. This was originally a freely available port of the C++ Standard Template Library (STL) as a Java library of packages. Now it is a for-sale extension of the Java Collections Framework to extend the set of data structures and algorithms available to the Java programmer by including the generic programming style of the C++ Standard Library.