[randomhack]

You forgot to mention Turing :) Turing doesnt get much space in school textbooks. And von Neumann too despite being one of the giants of mathematics in 20th century.

[mhartl]

Wow, my list really should have included von Neumann, perhaps the most underrated intellect of the 20th century. He'd be on a lot of people's top-n list even for n <= 3, and yet he's barely known outside of the technical world.

[brentr]

Let's add Alonzo Church to that list.