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

http://davesmechanicalpencils.blogspot.com/2006/06/guestbook.html?showCo...

http://groups.google.co.in/group/sci.engr.civil/msg/2f6c9150778f5036
Apparently some engineers prefer the Zebra M 301 and the Pentel P 205. I don't think I've tried these, but I suspect they might be heavier. That might be nicer for measured drawing, but I'm not sure I would enjoy the experience for light sketching and gliding writing.