Recommended: MLPowSim software

This site provides description of a free software package, MLPowSim, that calculates power for complex random effects models. It was developed by the Centre for Multilevel Modelling, the same group that developed the LMwiN package for analysis of complex random effects models.

Bristol University | Centre for Multilevel Modelling | MLPowSim. Available at: http://www.bristol.ac.uk/cmm/software/mlpowsim/