G*Power

G*Power is a free application that will run under Windows XP/Vista or Mac OS X 10.4. It uses both exact and approximate methods to calculate power. It will deal with samplesize/power calculations for t-tests, 1-way ANOVAs, regression, correlation, and chi-square goodness of fit. The program will calculate effect size for you after inputting some key information. For t-tests and ANOVAs you can find the effect size by supplying mean and variance information, for regression and correlation the effect size is a function of the r2. For all of these the non-centrality parameter of the appropriate distribution under the alternative is presented so that one can see how this effect size translates to these terms. There is some relatively extensive documentation on the web.