Evolus Pencil 3 was released recently. It is a GUI prototyping tool, suitable for mobile, desktop and web applications. I have used it on some projects to quickly put together screen views. Evolus Pencil is Open Source and is built onto the Electron platform.
- Page trees, subpages (good for reusing common background)
- Page links (also work in the exported HTML output)
- Flowchart symbols built in (with page links it's great to visualize user flow)
- Lots of primitve elements (form controls, tables, and so on) in different styles
- PDF and HTML exports
- Works on Windows, Linux and Mac