(1) Eclipse - *www.eclipse.org/
(2) NetBeans - *www.netbeans.org/
Eclipse and Netbeans are the two best although you will hear people raving about IDE's like JEdit, JCreator or the free version of JBuilder. The best one is Eclipse, but NetBeans is the only free one that currently supports the newest JDK 5.0
List of Java IDES
*alia.iwarp.com/java_ides.html
Open Source IDEs in Java:
*java-source.net/open-source/ides
www.intellij.net/eap