Web-based calculators
A list of all kinds of calculator links is provided by David Lane.
A t-distribution calculator I like gives and shows you the area under the t between any two points. It has slider bars for changing the boundaries and the degrees of freedom. It is pretty easy to use.
A similar calculator does the same as for the t-distribution but for the chi-square.
Hyperstat has calculators
for all kinds of distributions.
Explaining Hypothesis testing
There is a good explanation of the relationship between hypothesis testing and confidence limits Chapter 9 of Hyperstat.
A visual tutorial on type 1 and type 2 error and power of tests for the mean.