Charts, graphics, etc

http://d3js.org/ - JS library to generate graphics, charts, etc with any data.

Forms

Chosen: http://harvesthq.github.com/chosen/ - Enhanced and standardized form fields.
Formee: http://formee.org/ - Responsive form elements framework.

Responsive Web Design

http://www.responsinator.com/ - Online emulator (iPad, iPhone, Android, Kindle0

Sketching, prototyping, wireframing, etc

http://sneakpeekit.com/mobile-sketchsheets/ - Printable sketch sheets (including mobile).
http://www.mightymeta.co.uk/responsive-web-design-sketch-sheets/ - Responsive sketch sheets

CSS

http://nthmaster.com/ -> Graphic representation of different kinds of :nth-child formulas.