This EXCEL spreadsheet computes the power of an unpaired t-test assuming equal group sizes.

Inputs are the (two-tailed) type I error (alpha), the difference in group means, the average group standard deviation and the common group size. The program then outputs the power. The effect size used (the difference between group means in sds) is equivalent to specifying Cohen's d or Hedge's g.

This can also be done in R.