The greatest value of GeoGebra, or Geometer’s Sketchpad, or other such packages, in my eyes is their ability to help students dynamically visualize the effect each constant has on the graph of an equation. I find them an invaluable “thinking aid” as I ponder a new equation form, and they help me formulate my own answers to questions such as “why does it do that?”
Check out applets that help students explore the relationship between function parameters and their graphs:
Linear functions:
GeoGebraBook: Exploring Linear Functions, which contains
- Slope-Intercept Form – Interactive Linear Function Graph
- Point-Slope Form – Interactive Linear Function Graph
- Standard Form – Interactive Linear Function Graph
Quadratic functions:
GeoGebraBook of Quadratic Applets, which contains
- Factored Form – Interactive Quadratic Function Graph
- Standard Form – Interactive Quadratic Function Graph
- Standard Form – the effect of “b” – Interactive Function Graph
- Vertex Form – Interactive Quadratic Function Graph
Exponential and Logarithmic functions:
GeoGebraBook of Exponential and Logarithmic Applets, which contains
Rational functions:
Trigonometric functions:
GeoGebraBook for Exploring Trig Functions, which contains
Unit Circle Symmetries:
GeoGebraBook of Unit Circle Symmetry applets, which contains:
MathMaine 
Neat stuff. Have you discovered the Wolfram Alpha site yet? http://www.wolframalpha.com/
Yes! I have dabbled with it a bit, but have read a number of postings extolling its virtues while simultaneously wondering how to make best use of it in teaching.
Thanks for including my blog here. To the readers, lots and lots of step-by-step tutorials here:
http://math4allages.wordpress.com/geogebra/
you may want to check it out.