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