This is one of the best all around sites that I have found. It is titled “The MacTutor History of Mathematics Archive.”

http://www-history.mcs.st-and.ac.uk/~history/

This is an award-winning site that links to all sorts of mathematics information.

http://archives.math.utk.edu/

**Isaac Newton:**

http://www-history.mcs.st-and.ac.uk/~history/Mathematicians/Newton.html

**Gottfried Leibniz:**

http://www-history.mcs.st-and.ac.uk/~history/Mathematicians/Leibniz.html