Can we have a thread for sharing interesting or helpful websites relating to the fields of science?

I'll start with a few

https://www.ncei.noaa.gov/maps/oer-digital-atlas/mapsOE.htm
An atlas of various sea-floor mapping expeditions. Clicking on them provides more info and in some cases a link to the site of the expedition itself.

https://ptable.com/#Properties
Interactive periodic table. It can show the state of an element at various temperatures and other useful info in an easily presentable format.

https://apod.nasa.gov/apod/astropix.html
Good collection of astronomy-related pictures, every day a new one is featured with an explanation from an astronomer.