P2SM: https://ptonthat.fr/p2sm/ (sprites assets preparation, pixel art drawing)
MapEdit: https://ptonthat.fr/mapedit/
See the yt showreel and snapshots in the comments/changelog.
@sandor some YM stuff:
https://www.preromanbritain.com/maxymiser/
http://www.jchr.be/atari/ym2149.htm (French)
Other stuff:
https://github.com/ggnkua/half-sprite
http://ebook.pixas.de/index.php?frontpage
https://st-news.com/issues/st-news-volume-5-issue-2/education/optimizing-68000-code/