I use the Zebra Posi-Grip 0.5 M-1600 mechanical pencil for light sketches and writing equations. It "holds an edge" making it possible to do thick-thin calligraphy. The pencil is very light, but it glides smoothly free of squeaking and scratching. My friends and family hoarded this model from Kinko's about a decade ago before Kinko's became FedEx Office.

Unfortunately these pencils appear to have been discontinued. Here are a couple webpages mentioning it.
http://www.antipoetics.com/junk/pencils.html