Just a short update that I have created an unofficial list of public Matrix servers:

This list provides an unofficial overview over public Matrix servers. We only list servers whose owners have explicitly asked us to do so. If you want to appear on the list (or want to be removed from it), please join us in #hello-matrix:matrix.org to get in touch.

At the moment the list ist quite small, but I am hopeful that we will be able to add more and more servers to it in the coming weeks and months!