How to advertise a new game?

I made a small game and I don't know how to advertise it. Its open source and without charge. I released the web version and am working on the android and desktop (win, linux, maybe macOS) versions. Apple app store is way too expensive (99$/year) and steam also (99$ one time). I will never make any money out of it but would like to have some more people to know it.

Any ideas which are not too expensive?