Power On X is a tool for the calculation of statistical power. The program can perform calculations of required sample size to achieve a desired power, and calculations of power for previously run studies/experiments. In addition, the paid version can generate power/sample size tables.
STATISTICS STUDENT a good idea and nice start, but does less than g*power which is FREE! i know this is cocoa and precludes the need to start classic (unlike g*power) but $20 is way too steep. if the features get on par with g*power it will be worth $10. good luck! (Version 1.5.4)