How about models of computation?
- Turing machines
- Register machines
- Cellular automata
- $\mu$-recursive functions
- The untyped $\lambda$-calculus
- Unrestricted grammars
- Term-rewriting systems
- Recursively-enumerable subsets of $\mathbb{N}$
- $\ldots$
To this list we can add finite presentations of semigroups and groups, diophantine sets, and many other mathematical structures that realize universal computation.
