Isaac Newton, whose work on mechanics is still in use nearly 300 years after his death, was born on 25th December 1642. He is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution. His book Philosophiæ Naturalis Principia Mathematica ("Mathematical Principles of Natural Philosophy"), first published in 1687, laid the foundations for classical mechanics. Newton made seminal contributions to optics, and he shares credit with Gottfried Leibniz for the development of calculus.
And the best thing is - he really WAS born on 25th December, unlike the other chap whose birth date is not truly known and whose birth is celebrated on 25th December simply because the early church took over an existing Pagan festival on the same(ish) date.