1. Books. Every single masterpiece that was printed before… a certain date (it will depend the country you live in), is available for free online. They are not just ‘free’, they’re concentrated thoughts/cleverness/creativity available to anyone willing to take the time and, sadly nowadays, to make the effort of reading them.
2. Not really public domain but close enough: public libraries, most of them will be free and they will also let you access non-public domain books too, even the most recent titles.

For me, it’s an everyday little miracle that publishers have not yet managed to convince lawmakers public libraries should be made illegal… because a lot of those publishers only want us to buy their books, much more than they want us to read them.

Public libraries are one of the most undervalued resource in many countries. One day, they will be gone, and a majority of us will not even care. Sad.

edit: typos.