The complete ImageJ source code.
This command takes you to the colourized browsable ImageJ source code page: http://rsb.info.nih.gov/ij/developer/source/
That page uses frames. Note that the browsable source code may not be the latest version.
The latest zipped source code can be accessed through Help>ImageJ News menu command. Follow the Upgrade link and then the Java source code link which takes you to:http://rsb.info.nih.gov/ij/download/src/