Sometimes a 0.5mm mechanical pencil, but more often an archive-quality felt tip art pen. The Pigma Microns are awesome, though I've currently got a Faber Castell in my pocket. That gets used for everything that goes in my notebooks - sketches, mechanical diagrams, writing, you name it. I find it's a nicer feel, less bleed, and much more consistent line compared to ball-points, particularly on the pack of index cards that I carry in my pocket for notes and to-do lists.