Today I noticed this question, and I thought it would be helpful if we had a thread that listed resources that people could conveniently access for power analysis / sample size calculations, perhaps analogous to this thread: Resources for learning R.
Answer
Power analysis refers to analytical procedures that attempt to determine the power of a statistical test (i.e., the probability of rejecting a false null hypothesis) or the sample size (i.e., N) required to achieve a given power. You can search Cross Validated for more information about power analysis by clicking here: poweranalysis, here: power, or possibly here samplesize.
Free software for power analyses:
 G*Power
 PS: Power and sample size calculator
 Optimal Design software – statistical power analysis for multilevel and longitudinal research
Websites with free sample size calculators:
 Russ Lenth has a javabased web app
 Michael Friendly has a power analysis / sample size calculator for ANOVA designs
 Rollin Brant’s webpage for comparing two proportions
 OpenEpi has sample size/power calculators for basic epidemiological study designs.
 webpower a web based interface for power analyses (based on the package listed below) for people who don’t use R
Open source programming languages with power analysis libraries / modules:

R packages:
 pwr is the oldest poweranalysis library; some introductory info can be found on QuickR
 PoweR: Computation of power and level tables for hypothesis tests
 Power2Stage: Power and Sample size distribution of 2stage BE studies via simulations
 powerAnalysis: Power analysis in experimental design
 powerGWASinteraction: Power Calculations for Interactions for GWAS
 powerMediation: Power/Sample size calculation for mediation analysis, simple linear regression, logistic regression, or longitudinal study
 powerpkg: Power analyses for the affected sib pair and the TDT design
 powerSurvEpi: Power and sample size calculation for survival analysis of epidemiological studies
 PowerTOST: Power and Sample size based on two onesided ttests (TOST) for (bio)equivalence studies
 longpower: Power and sample size for linear model of longitudinal data
 webpower: “a collection of tools for conducting both basic and advanced statistical power analysis” (also a website)

There is a Python module, statsmodels, that can run some power analyses. There is some information here.
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