What's the best free alternative to Omnigraffle?

Ralph Garcia recommended Visual Paradigm Online as the best free Omnigraffle alternative.

According to Ralph, Visual Paradigm Online supports a wide variety of diagram types such as AWS diagram, Azure Architecture diagram, UML, BPMN, ERD, DFD, flowchart, ArchiMate, organization chart, mind map, venn diagram, strategic analysis (e.g. SWOT, PESTLE, etc), etc. Here are some excellent features they offer:

• Drag and drop to create shape
• Alignment guide
• Wide range of formatting tools
• Custom shape library
• Export diagram as PNG, JPG, SVG, GIF, PDF
• Real-time synchronization
• Diagram layers
• Visio drawing and stencil import
• Google drive integration

Ralph said that VP Online offer an Express Edition that allows you to draw these diagrams free: UML, ERD, Organization Chart, Business Concept, ITIL.