The following sample size calculators are for free:
1. PS (http://biostat.mc.vanderbilt.edu/twiki/bin/view/Main/PowerSampleSize)
2. Russ Lenth's on-line sample size calculators
(http://www.stat.uiowa.edu/~rlenth/Power/)
3. Open-EPI
(http://www.openepi.com/Menu/OpenEpiMenu.htm)
4. David Schoenfeld's Statistical COnsiderations for Clinical Trials and Experiments
http://hedwig.mgh.harvard.edu/sample_size/size.html
5. SISA (Simple Interactive Statistical Analysis)
http://www.quantitativeskills.com/sisa/calculations/samsize.htm
