In various attempts to try new strategies, I’ve often thought of doing adaptations. Working with characters, settings, and stories that are already written might help free your creativity up to experiment and play.
However, adaptations of modern works are (for the most part) illegal to produce for profit, so while they may still be a valuable exercise, they’re not the best career move. Because of this, I decided to look up works in the public domain–works that can be used and even profited from, if applicable.
I am obviously not a copyright lawyer, so I don’t know the specific rules here, but generally anything published before 1923 (at least when I made this list a couple years ago) is no longer under copyright. More recent works may be out of copyright in certain circumstances, but the laws have changed over the decades. If in doubt, consult multiple sources or even a copyright lawyer before pursuing publication.
So here is a list of works in the public domain–certainly not exhaustive, and with some more vague “categories” intended to stir up more material.