Couleurs is a simple app for grabbing and tweaking the colors you see on your screen.
Couleurs takes Apple's color panel (complete with "magnifying glass"), and combines it with an easy way to copy common formats of color values to the clipboard (including CSS formats like #rrggbb and even UIColor values in Swift).

SVG icons for popular brands, maintained by Dan Leech. Contributions, corrections & requests can be made on GitHub.

A cross-platform design tool for the 21st century
Gravit Designer is a full featured free vector design app right at your fingertip.

Pixel Winch is a screen measurement app with a unique approach. It combines aspects of a traditional image editor with the quick access of a modal interface (similar to macOS's Launchpad, Mission Control, or Dashboard).

Tufte CSS provides tools to style web articles using the ideas demonstrated by Edward Tufte’s books and handouts. Tufte’s style is known for its simplicity, extensive use of sidenotes, tight integration of graphics with text, and carefully chosen typography.