Power and Sample size calculations:

There are a lot of free downloadable sample size and power calculation software packages.  This is one of my favorites:

http://biostat.mc.vanderbilt.edu/twiki/bin/view/Main/PowerSampleSize

An easy to use web-based set of calculators: http://www.stat.ubc.ca/~rollin/stats/ssize/

Please add you favorite to the list.