SpeedCrunch is a fast, high precision and powerful desktop calculator

SpeedCrunch
Free
Offline
http://www.speedcrunch.org/

Speedcrunch is a high precision and high speed calculator. It's optimized for keyboard use and has advanced features: use of functions, use of variables, result history, and syntax highlighting. it also shows the result as you type. Speedcrunch has a very simple interface, so you can start to use it very quickly.