Damas-Milner type inference (often called Hindley-Milner type inference) was published in 1983 and has been the basis of every sophisticated static type system since. It was a genuinely new idea in programming languages (admitted based on ideas published in the 1970s, but not made practical until after 1980). In terms of importance I put it up with Self and the techniques used to implement Self; in terms of influence it has no peer. (The rest of the OO world is still doing variations on Smalltalk or Simula.)
Variations on type inference are still playing out; the variation I would single out the most is Wadler and Blott's type class mechanism for resolving overloading, which was later discovered to offer very powerful mechanisms for programming at the type level. The end to this story is still being written.
