Strip Generator
Quick comic strip tool for storyboarding.
Skitch
Mobile sketching and annotating app with Evernote integration.
POP
Mobile prototyping app for building simple functionality with hand-drawn sketches.
Wacom Inkling
http://www.wacom.eu/index2.asp?lang=en&pid=9226&gm=4
Clip-on device for converting hand-drawn notes to digital files.
Balsamiq
Sketching tool for rapid wireframing.
Paper
http://www.fiftythree.com/paper
Virtual notebook sketching app for iPad.
iBrainstorm App
http://www.ibrainstormapp.com/
Virtual post-it/sketching app for connected iPhones and iPad.