Thomas Bayes

Developed the interpretation of probability now known as Bayes theorem, one of the basic theorems in probability theory
(which is also successfully used for spam filtration purposes).

Carl Friedrich Gauss

Invented least squares estimation methods, which are still widely used in social sciences statistical applications,
particularly in regression analysis and modeling.

Adolphe Quetelet

Astronomist, mathematician, statistician and sociologist, who pioneered the use of probability and statistics in the social sciences.

Francis Galton

Invented and first introduced the concepts of standard deviation, correlation and regression (key things for social science statistics).

Karl Pearson

Famous for manyinnovations and inventions, including the development of the Pearson chi-squared test and the Pearson correlation.

Charles Spearman

Extended the Pearson correlation coefficient to the Spearman's rank correlation coefficient - relatively little but important contribution :).

William Sealy Gosset (known as "Student")

Discovered the Student t distribution and invented the Student's t-test.

Gertrude Mary Cox

Has done a lot on Experimental design, and was a first woman elected into the International Statistical Institute (1949).

Ronald A. Fisher

Carlo Emilio Bonferroni

Invented the Bonferroni correction for multiple comparisons coefficient - relatively little but important contribution :).

Jerzy Neyman

Discovered the confidence interval and co-developed the Neyman-Pearson lemma.

Egon Pearson

Co-developed the Neyman-Pearson lemma of statistical hypothesis testing (people ofter think that it was the other Pearson).

Maurice Kendall

Co-developed methods for assessing statistical randomness; invented Kendall tau rank correlation coefficient.

John Wilder Tukey

Pioneered in exploratory data analysis and graphical presentation of data, developed the jackknife for variance estimation, invented the box plot(!).