CoolCalc
CoolCalc is a simple calulator what evaluates expressions. It does not try to mimic the way you work with a pocket calculator but rather work on a more intuitive level.

Screenshot
See the help file for more information on what it is about.
Download:
Installation:
Just extract it and execute. No installation required. Likewise, no fancy un-installation provided - just delete it :-)
The enclosed CoolCalc.html shall be in the same dir. as the .exe for the Help to work.
Source:
Source (Visual C++ 6 project) is available for the curious, let me know if you want it.