Newton

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… See the whole entry

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/

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

Leibniz:… See the whole entry