Calculate with Qalculate on Linux
The calculator options on Linux just blows the Windows calculator away. Imagine a calculator where you can solve extremely complicated expressions, or just convert between different measurements, and you've got Qalculate.
You can use it to convert currencies based on the latest exchange rates, which the application will download for you.
You can even solve extremely complex algebra functions.
You can even use it to convert between a unix timestamp and regular dates.