Wolfram Research
Wolfram Math World: World's Most Extensive Mathematics Resource
MathWorld provides a huge mathematics encyclopedia detailing the concepts of geometry, algebra, calculus, discrete math, number theory, and other math topics--complete with formulas, diagrams, examples, and references for thorough...
Trinity College Dublin
History of Mathematics: Bonaventura Cavalieri (1598 1647)
A biography of Cavalieri that includes information on his principle or "indivisibles." From a 1904 text by W. W. Rouse Ball.
University of St. Andrews (UK)
University of St. Andrews: Johann Peter Gustav Lejeune Dirichlet
A biography of Dirichlet's personal life and on his mathematical findings, mainly in calculus and number theory.
Other
Academic Info: Mathematics Gateway
This site provides a comprehensive index and general directory to math sites that will help students in the study of circles.
Other
Seton Hall University: Johann Bernoulli (1667 1748)
A great biography of Johann Bernoulli is provided at this site. An audio clip is provided, as well as a good list of primary sources.
University of St. Andrews (UK)
University of St. Andrews: Nicolaus(ii) Bernoulli
This site, which is provided for by the University of St. Andrews, gives a brief biography of Nicolaus Bernoulli.