Tools to help retain knowledge from coursework?
Unless your own university provides you with such software, it would probably be too hard for the software to know what topics you have studied exactly, and in how much detail. But you are right: if you do not work with your knowledge on a daily basis you will forget things. But this is not a problem. The "magic" solution you are looking for is called a job :).

Inevitably you will forget some of the things you do not work with every day. But your education will provide you with enough knowledge to look up the answers you need, and quickly be up to speed again.

The way I see one of the main goals of an MSc is to be able to read the scientific books and papers in your own field, and the fields close to it. This is how you will refresh and update your knowledge constantly when you start working in science.