AskHistorians

The (now) proverbial soap box.

Other than that, nothing. In the past, newspapers were basically the only source of information and opinion with regional, national or international reach.
That's why the freedom of the press is so important.

There were some public or semi-public billboards where you could post leaflets.

But only the invention of the internet allowed private citizens without any financial capital to broadcast to millions, if their message was catchy enough.