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:
- LIKO-12 - based on the Lua programming language
- neko8 - also Lua
- PX8 - Python3/Lua
- Riko4 - Lua
- TIC-80 - Lua/Moonscript/JavaScript(?)
- VectorBoy - Lua
- Zenith80 - Z80 Assembly/C (now that's interesting!)
- Tako 80 - JavaScript
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.