Some Mathematical Morphology plugins are available on the main ImageJ web-site:

Morphology operators for:

• Gray-scale images: http://rsb.info.nih.gov/ij/plugins/gray-morphology.html
• Float images: http://rsb.info.nih.gov/ij/plugins/float-morphology.html

Granulometry: http://rsb.info.nih.gov/ij/plugins/granulometry.html

Summary and API documentation of the plugins can be found here: http://www.diagnosticarea.com/plugins.html