#TypeCalc
Spotlight has a great builtin calculator, but it doesn't keep a history of previous expressions and results, also the window disappears when you switch between applications.
TypeCalc is a small application that basically doing same job as Spotlight's calculator, but it also records all previous expressions/results and app's window stays in place.
Currently, this application uses Calculate.framework to parse and evaluate arithmetic expressions.
Tested on macOS 10.11-11.0