• GeoGebra - A free (perhaps better) alternative to Geometer's Sketchpad. Current version is 3. There is a beta for version 4. There is also another beta for version 5 that allows 3D constructions.
  • MathGV - Fast little program that I used for graphing before I found GeoGebra
  • Maxima - A computer algebra system useful for manipulation of symbolic and numerical expressions
  • Octave - A free alternative to MATLAB
  • SAGE (aka SageMath) - A free alternative to Mathematica/MATLAB
  • Touch Mathematics (Trigonometry) - Interactive flash that shows connection between the graph of trigonometric functions and the unit circle
  • Wolfram Alpha - An answer engine with Mathematica backend that allows natural language queries.