What are the most interesting software calculators you have found?

Not the usual physical scientific calculator emulators, calculators that bring something novel.

Examples:

https://victorpoughon.github.io/interval-calculator/

calculator++ (android), for typing by swiping on keys and all its features.

Sticky calculator (https://apps.microsoft.com/detail/9nblggh4svpt?hl=en-US&gl=AR), sleekest, best UX ever.