The Turing machine, first described in Alan Turing's classic paper "On Computable Numbers", is a seminal thought experiment that led directly to the machine you're currently using to read this. The Turing machine was never really meant to be built, but now some guy (not an academic, from what I can tell) has gone and built one, and it's capable of performing actual computations. Awesome.
Note: I've discussed Turing (once) before, and noted he had some daft ideas...
