Unleash - Open source Feature toggle/flag service. Helps developers decrease their time-to-market and to increase learning through experimentation.
Facebook Design - Devices - Images and Sketch files of popular devices.
Marvel for iOS - Rapid design and prototyping for the iPhone and iPad
Smartmockups - Create stunning product mockups, easily and online.
Google Sites - Access Google Sites with a free Google account (for personal use) or G Suite account (for business use).
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.