rca is simple yet versatile
It's a reverse polish notation calculator in the HP tradition
It seamlessly evaluates infix-style expressions
It runs in either floating point or integer mode
It has the trig and math functions of a scientific calculator
It has the word width selection and bitwise operators of a
programmer's calculator
It supports variables
It can be trivially incorporated into shell scripts
It's written in C, and maintained in a single source file
rca isn't an an arbitrary precision calculator like dc or bc.
It's a convenient, non-programmable, desktop calculator, for
typical one-off, everyday problems.
The man page and rca's help text have all the details.
rca is hosted at github.
The license can be found in the source or viewed with rca's
"license" command. (It's BSD-2-Clause.)