A big list of unlikely or surprising Turing-complete systems: https://www.gwern.net/Turing-complete, via https://www.metafilter.com/183095/On-having-sufficient-complexity-to-allow-for-arbitrary-computation

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): https://github.com/tom-p-reichel/svg-is-turing-complete