Wolfram Research
Wolfram Math World: Radius of Convergence
MathWorld provides a site about the Radius of Convergence. Brief and factual, formulas are provided along with several links to additional information on related subjects.
Khan Academy
Khan Academy: Alternating Series Test
Practice your understanding of the alternating series test and determine whether given series converge conditionally or absolutely.
Other
Shippensburg University: The Root Test
This site provides a brief overview of the root test, presenting one sequence with converges absolutely and one which diverges.