Origami is a free toolkit for Quartz Composer—created by the Facebook Design team—that makes interactive design prototyping easy and doesn’t require programming.
The Colour Contrast Check Tool allows to specify a foreground and a background colour and determine if they provide enough of a contrast "when viewed by someone having color deficits or when viewed on a black and white screen".
ish. is yet another viewport resizing tool. It’s called “ish.” because it focuses on general ranges (small-ish. medium-ish. large-ish.), rather than popular device widths.