Once in a while one has to determine the power of an experiment, the effect size, or the adequate sample size. Nearly all statistical programs allow to determine the effect size of an experiment and/or its power. But why powering up SPSS, when a small tool would do as well if not better?
G*Power 3 offers different types of statistical power analysis. Typically one uses a-priori or post-hoc. G*Power 3 is available for Mac OS X 10.4 and Windows XP/Vista. It has been described in
Faul, F., Erdfelder, E., Lang, A., & Buchner, A. (2007). G*Power 3: A flexible statistical power analysis program for the social, behavioral, and biomedical sciences. Behavior Research Methods, 39(2), 175-191.
It can be downloaded here.