Calculon is far and away my favorite Mac calculator. Small, simple, and to the point. No silly number keys to click like all the other calculators out there (take a look down at your keyboard for a much more effective alternative if you find yourself missing them).

The display can be cleared by pressing the "C" key.

** Notes for the maintainer **
Homepage should be:
http://pixpeep.com/calculon/

Download link should be:
http://pixpeep.com/calculon/calculon.zip

I would also recommend adding a "widget" keyword.