Blue Pencil is a plug-in for Maya that brings 2D drawing to the 3D viewport.
Blue Pencil is a Sketchpad for Maya is now available for Maya 2011 SP1, Maya 2012 and Maya 2013 on Windows and Mac OS X. A Linux version is currently in beta testing and nearing release.
It is a multi-purpose tool that is useful for, but not limited to:
Blue Pencil is presenting itself as a Grease Pencil, for planning, annotation and animation, creating thumbnails, rough blocking and arc tracking.
Blue Pencil can also be used for Model layout and topology planning as well as a shot review tool. Can be used for tutorials and screen-casting as well.
Blue Pencil provides pressure-sensitive Wacom support for line thickness in the pencil tool, and opacity in the brush tool, along with an eraser tool and eyedropper. All drawing operations have native Undo/Redo support and the drawing data is stored directly in the Maya scene file.
From an animation standpoint, keyframes are integrated directly into Maya's timeline (for easy re-timing), keys can be cut, copied and pasted, and the Lightbox (onion skinning) can be used to see pre and post keys.