the sample size site which mentioned by Nonglak is in the following link

http://www.ncss.com/software/pass/

hi, one of the most common available free progrmas is the G.power. its size is around 10mb. it is been built based on Cohen power for sampling.

one of the links to donwload is 

http://download.cnet.com/G-Power/3000-2054_4-10647044.html

Dr. Ahmed greetings.

The bluk of available literature is discussing sexual dysfunction and combined pill. Up to my knowlege no

previoud studies had examined this relationship

G power soft ware for sample size calculation foe windows and Mac from Germany

Sample size determination can take various approaches. In addition to G-power as already mentioned, you may also look at the paper attached. Hope it helps.

http://stat.ubc.ca/~rollin/stats/ssize/

Try this website 

http://www.raosoft.com/samplesize.html

I used to work on the link posted by Ahmad Deeb

Salam Dr

Try this one

http://www.surveysystem.com/sscalc.htm

see here

http://www.openepi.com/Menu/OE_Menu.htm

The application "n4 Studies" , you can download it from App store for free.

 the sample size site which mentioned by Nonglak is in the following link

http://www.ncss.com/software/pass/

There are many free software for sample size calculations