Wolfram Research
Wolfram Math World: Parabola
This is an excellent site on parabolas. Examples and formulas are given.
University of Illinois
University of Illinois Urbana Champaign: Egg Math: Spherical Geometry
This site contains an experiment that demonstrates how transporting a tangent vector around a closed path on a sphere will cause it to come back twisted. [Requires Java.]
Texas Instruments
Texas Instruments: The Tangent Ratio
In this activity students explore the tangent ratio. Students construct a right triangle using perpendicular lines, and a line that forms the hypotenuse of the triangle. When students construct the hypotenuse line it is very important...