There are a few out there but you usually don't get as much as you hope for from a used equipment company. In the past, I have donated Modicon PLC equipment to my University. They have a ladder logic course in the Chemical Engineering Department. The tax right off was better then what I was offered by used equipment companies.
I took the controls course and later both donated PLCs and lectured in the course. If you decide to donate and there isn't some place local, I would be happy to put you in touch with someone at UML who could help you. I just don't end up with equipment I can donate to them any more. Where are you located?