Get Shit Done - Free Bootstrap 3 UI kit
Invision - Prototyping and collaboration for design teams
NOW UI kit by InVision - A beautiful cross-platform UI kit (PSD & Sketch)
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Spaces by proto.io - A community to showcase your interactive prototypes
Craftwork - A collection of User Interface resources made by Craftwork