Qalculate!

Qalculate! is a multi-purpose cross-platform desktop calculator. It is simple
to use but provides power and versatility normally reserved for complicated
math packages, as well as useful tools for everyday needs (such as currency
conversion and percent calculation). Features include a large library of
customizable functions, unit calculations and conversion, physical constants,
symbolic calculations (including integrals and equations), arbitrary precision,
uncertainty propagation, interval arithmetic, plotting, and a user-friendly
interface.