What are the most reputable on-line sources for written medical
information? I am developing training materials for Social Workers who
work with amputees.  I want to be able to quote reputable sources for
the information I include in the materials.

Depends on what sort of info you are looking for and how detailed do
you want it to be. There are so many websites out there. But a good
one (and which is not free) is www.uptodate.com. They do give you a
few days trial subscription and that should help. Another site
ofcourse, is www.medscape.com or webmd.
If you  need any more specific information, let me know.

I find The Mayo Clinic has useful and reliable information:
http://www.mayoclinic.com/
Also the National Institute of Health's Medline Plus:
http://www.nlm.nih.gov/medlineplus/medlineplus.html