The Soulver app for iOS is a great mix of notepad and calculator. You can take notes with calculations in them extremely easily.
Write down anything in the notepad, shopping lists, budgets etc, and see calculations in real-time.
The app doesn’t look like much when you first open it (just a blank text area), but there are many hidden features that are very powerful.
Soulver has many hidden features, such as binary and hexadecimal number support, currency conversion, math functions, variables, etc.