Diagramo - Diagramo is an online flowchart software - diagrams (create, edit and share)
Now UI Kit - A beautiful Bootstrap 4 UI kit. Yours free.
Modelio - Modelio is an open source modeling tool supporting natively UML, BPMN and model exchange with XMI.
Bots UI Kit - Fully customizable Sketch UI Kit for Messenger Platform
UML Designer - UML Designer provides a set of common diagrams to work with UML 2.5 models.
Landing UI Kit - Handcrafted UI components for landing pages