Hi There,
Java Studio Creator and Netbeans are the 2 free and extremely popular IDEs for Java Development from Sun.
If you are a newbie to Java or just want to do rapid web Application development use Java Studio Creator. But if you want to create more complex J2EE Enterprise Applications, Netbeans would be your choice.
The links are
Java Studio Creator - http://developers.sun.com/prodtech/javatools/jscreator/index.jsp
Netbeans -
www.netbeans.org
Hope this helps.
K