Wikipedia
DrJava
DrJava is a lightweight Java IDE designed primarily for students and beginners in Java that is actively developed and maintained by the JavaPLT group at Rice University. Its interface has been developed using Sun Microsystems' Swing toolkit and therefore it has a consistent appearance on different platforms. DrJava has the ability to interactively evaluate Java code from a console and to present output as well to the same console. It has many other features that have been designed for advanced users as well. DrJava offers a JUnit test facility.
DrJava reached the milestone of two million downloads of DrJava. There have been 2,204,443 downloads as of midnight, 25 Nov 2013.