This resource explains margin of error using an example on presidential popularity polls.
This resource gives 3 questions readers should ask when presented with data and why to ask them: Where did the data come from? Have the data been peer-reviewed? How were the data collected? This page also describes why readers should: be skeptical when dealing with comparisons, and be aware of numbers taken out of context.
This page calculates probabilities for a Poisson distribution.
This page calculates either estimates of sample size or power for differences in proportions. The program allows for unequal sample size allocation between the two groups.
This applet displays various distributions and allows the user to experiment with the parameters to see the effects on the curve.