Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
Paste by FiftyThree - Show your work. Paste is a new presentation tool for Slack.
Facebook Design - Resources for Designers from the Facebook Design team
Inspect by InVision - A magical new design to development workflow from InVision
InVision Studio - The world’s most powerful screen design tool
Design Principles - An open source repository of design principles and methods