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