Someone on the Apple Java list told me about BlueJ, a nice beginner IDE for student java developers. What's cool abut it is that it was written as an environment for learning java instead of working with java, so it's a lot simpler than using say ProjectBuilder or Netbeans.

Not to say I don't want to use them, but this has some very nice features, including the best compiler error handing I've ever experienced... not only does it tell you the error, but sometimes gives a hint as to what's wrong.

Written entirely in java, there are Mac OSX, Windows and Unix versions (the OS X version has the Aqua L&F).