I've designed and developed from scratch a web-based calculator, which automates a number of calculations necessary while working with metrics and A/B-tests — prodcalc.app.
When one measures any metric it is necessary to check if the result has statistical significance. Prodcalc calculates confidential interval for a measurement on a sample, as well as compares two measurements, so product manager could determine if the difference between those measurements has any statistical significance.
Often one faces an inverse problem—how many people are required for the test, so the result has predetermined confidential interval. Prodcalc automates these calculations, too. Also, on the same page, there are calculations for an A/B-test sample size.