One who thinks that all human knowledge is gained through experience. This bit of epistemology is known, believe it or not, as Empiricism. John Locke, David Hume, and George Berkeley* were empiricists. John Stuart Mill was even more of one, as he did not even believe that the rules of math and logic could be known a priori.

See also Logical Positivism, Positivism, Verification Criteria of Meaning

If all of this sounds terribly wrong to you, you might be a Rationalist.

If those weren't enough for you, here's a list of some other famous empiricists.
Willard van Orman Quine
William James
Bertrand Russell
Clarence Irving Lewis
Rudolf Carnap
A.J. Ayer (Alfred Jules Ayer)