Software

Behavioural Calculus
Offers a range of functionality, including a scientific calculator, function plotting in two and three dimensions, and problem solving. Screen-shots and details of features. Commercial, but free demo version available to download.
http://www.orfeobernardo.com/bc/

Calculus Applets at St. Louis University
Provides applets demonstrating concepts of both single and multivariable calculus.
http://www.slu.edu/classes/maymk/MathApplets-SLU.html

Calculus Made Easy for the Ti89/Titanium/Ti-92+/Voyage200
Collection of subprograms and menus that will help the user solve calculus problems and recall useful theorems, definitions, and rules.
http://www.islands.vi/~shawn/ti_89.htm

Calculus Problem Solver
A calculus tutorial software that can solve differentiation problems and generate step-by-step solutions. Tests differentiation skills by generating dynamic quizzes.
http://www.runiter.org/calculus.htm

GraphFunc
A program online that is used to plot the graph of mathematical functions in two or three dimensions. [Java required]
http://graph.seriesmathstudy.com/

SID2002 Math Expert System
Solves problems and shows step by step solution using artificial intelligence.
http://www.geocities.com/ashtry_sid2002/

SymPy
A Python library for symbolic mathematics.
http://code.google.com/p/sympy/

Topics in Integral and Differential Calculus
Contains several applets illustrating fundamental concepts of single-variable calculus.
http://www.ma.utexas.edu/calculus/

Wolfram Mathematica Online Integrator
Allows users to enter an integrand in order to return the indefinite integral.
http://integrals.wolfram.com/index.jsp