University of St. Andrews (UK)
University of St. Andrews: Leonhard Euler
Lengthy biography of Euler that sets him in relationship with his contemporaries in mathematics and physics. Several quotes and pictures available, as well as links to relevant mathematical topics. A terrific site.
Wolfram Research
Wolfram Math World: Ratio Test
This page states the Ratio Test for convergent or divergent series, sometimes called the Cauchy Ratio Test or the d'Alembert Ratio Test. Brief but informational.
Other
U of North Dakota: Fundamental Theorem of Algebra
A historical overview of the Fundamental Theorem of Algebra. Links to historic mathematicians and the development of this theorem.