A web page that calculates the chance of hatching the pokémon with the stats you want. Includes a complete pokédex of breedable pokémon. It can calculate for IVs, natures, abilities, gender, and shininess with a variety of held items.
You have a few choices!
If you're good at code, you can check the issues page for stuff that needs doing, or you could download the source and take a crack at adding a new feature. If you're extra ambitious, you could start a new branch for an earlier generation of pokémon.
If you like to breed pokémon, you can help make sure the statistics are right by hatching a few boxes of pokemon. You can see the tests and comment your results on the google sheet.
If you noticed a bug, you can open an issue on the issues page. Be sure to include what species of pokemon you're working with, what other settings you used, what you expected to happen, and what happened instead.