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I've been researching open-source alternatives to PICO-8, since I really like the idea of limiting  oneself in order to boost one's creativity. Besides, it perfectly fits into the theme of the jam. So here's a list of what I've found so far:

They are all very similar, except for the VectorBoy which is (as the name suggests) for vector graphics, and Zenith80 which is for hardcore programming :)
As far as the functionality, stability and documentation go, it looks like TIC-80 and PX8 are the most mature, and they also have the biggest communities, but I am eager to try less known ones.