Figma - Team-based interface design, Figma lets you collaborate on designs in real time.
Design for Startup - Everything about design for startups
Facebook Design - Resources for Designers from the Facebook Design team
Designing UX for SaaS - Free ebook to level up the User Experience of your SaaS
InVision Studio - The world’s most powerful screen design tool
Design Principles - An open source repository of design principles and methods