Made some great course slides, now what?
Here are some ideas:

Make a video of yourself presenting the slides (either to a class, or just to the camera), and upload it to YouTube (or similar). Others have done this, and as a result there are some great free resources available for people who otherwise would not have the time or money to take the course. For example, N J Wildberger has several maths courses online. If you don't want to be on the video, you can just have the slides, with your voice presenting them. (See the videos of Bill Shillito, for example.)

Put the materials on your website, and spread the news.

Turn the materials into a wikibook, where others can add to them.

Put the materials on OER Commons, Curriki, MyOER, or Share My Lesson. I haven't used any of these myself (yet), so I don't know which would be most appropriate for university-level or postgraduate teaching materials.