Sample size calculator

Statistics can be very confusing and often data is interpreted wrong. Especially when it comes to A/B testing and sample size. Here some of the tools/sites that I like:
Sample Size Calculator via Surveysystem
Java applets for power and sample size via Ruth Lenth

Inference of two Sample proportion by Rebecca Su

Frequency Stats via Swogstat

More tools are in our tools section. Please feel free to add any tools that help your daily work