Apparently this used to cost back in the day but since then the devs made it and the 70ish asset packs free (excluding a couple because the original artists were stingy). As far as game maker engines goes it's complete shit but stumbling across this when I was small had a massive hand in forming my appreciation for game development. As well there used to be a bit of a community surrounding this engine with some popular mods including the Black Ice multiplayer mod (might have botched the name).
For a kid's first encounter with proper game dev this engine is actually pretty sick.
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Oh and as well there's M.U.G.E.N
If you like fighting games or like any character from anything ever I'm sure you can find some enjoyment in M.U.G.E.N. Even gamblers love it because of Salty Bet
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Btw, awesome thread, OP. I only clicked on to see what it was but this is actually a super cool list to have so sorry if these don't count and I messed it up