Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
Invision - Prototyping and collaboration for design teams
Dashboard UI Kit - A modern & responsive dashboard UI kit for designers.
Mockuuups Studio - Fast and easy way to create product mockups on macOS, Windows and Linux.
Landing UI Kit - Handcrafted UI components for landing pages
Craftwork - A collection of User Interface resources made by Craftwork