Precursor - Precursor helps designers and developers communicate and collaborate in real time.
Facebook Design - Devices - Images and Sketch files of popular devices.
mi.team - Collaboration app with chat, task board & video conference
Clay Mockups - Free set of beautiful device mockups for your designs
Justinmind - Define awesome experiences through wireframes & prototypes.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.