Qalculate! is a multi-purpose desktop calculator for GNU/Linux (now ported to Mac via MacPorts). It is small and simple to use but with much power and versatility underneath. Features include customizable functions, units, arbitrary precision, plotting, and a user-friendly interface (GTK+).
Qalculate! has now moved to GitHub (https://github.com/Qalculate). Get the latest version (0.9.10) from http://qalculate.github.io/downloads.html.
There are no 1 star reviews.