A modified version of the original Greenfoot Java IDE.
In the bluej/docs folder there will be a text file with the extension .how-to-open-bluej-in-eclipse or something similar. Open the document with wordpad or Microsoft Word. Follow the instructions exactly as written. Then go to greenfoot/docs and another text file with an extension similar to the one you followed for bluej. Follow all of these directions and it will setup the source code in eclipse.
When you have completed all the directions, greenfoot should run properly. Any edits you make to the source code should appear when you run the program.
There is a compiled and ready-to-run collection of installers for various systems in the releases section.