Online calculation for sample size, power or the minimal detectable difference for parallel studies, crossover studies, or studies to find associations between variables, where the dependent variable is Success or Failure, a Quantitative Measurement, or a time to an event such as a survival time. Registration not required.
This page from RV Lenth provides graphical interfaces for studying the power of one or more tests. They include sliders (convertible to number-entry fields) for varying parameters, and a simple provision for graphing one variable against another. Registration not required.