From Department of Psychology:
G*Power 3 is a statistical power analysis program designed to analyze different types of power and compute size with graphics options. It covers many different statistical tests of the F, t, chi-square, and z test families as well as some exact tests. G*Power 3 provides improved effect size calculators and graphics options, it supports both a distribution-based and a design-based input mode, and it offers five different types of power analyses. G*Power 3 is free. Version 3.0.5 is a bug fixing release.