Clac is a [rpn][rpnWiki] calculator made to live inside your terminal. If you pass it an expression, it will evaluate it in place and let you go on your merry way.
If, however, you don't give it an expression, it opens up and interpreter for you to use for hardcore calculating.
Based on the 15-122 project at CMU, clac is meant to be fast, easy to use, and easy to understand.
To install: ghc clac
Then move the clac binary to somewhere in your PATH. [rpnWiki]: https://en.wikipedia.org/wiki/Reverse_Polish_notation