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A big list of unlikely or surprising Turing-complete systems: gwern.net/Turing-complete, via metafilter.com/183095/On-havin

My favorite: SVG is Turing-complete because it can be used to (slowly) simulate Rule 110 (and one hopes the weird boundary conditions needed to make Rule 110 Turing complete): github.com/tom-p-reichel/svg-i