The philosopher Dan Dennett, in his recent book Intuition Pumps and Other Tools For Thinking, devotes a rather large section to URMs titled The Seven Secrets Of Computing Power Revealed.

I will demo a DSL, that is an interesting use of higher order functions, for a kind of URM . The hope is that by simulating the simplest possible register machine and building up complex algorithms you can understand what it means to compute, how a computer works and along the way we will prove the undecidability of the Halting Problem.