TOOLS: Pencil (rapid Prototyping tools - for mockups creation as well as live feature navigation)

A tool has been pointed out to me (free at the time of posting) and requiring installation (as opposed to some similar web-based tools):
https://pencil.evolus.vn/

Quoting:
"An open-source GUI prototyping tool that's available for ALL platforms.
Pencil is built for the purpose of providing a free and open-source GUI prototyping tool that people can easily install and use to create mockups in popular desktop platforms.
The latest stable version of Pencil is 3.0.4 which contains stability fixes and features a visual stencil builder. More details can be found in the release notes."

PS. About rapid prototyping (tools):
On Software Development (or "O Sr. SiIva disse..."): TOOLS: Rapid prototyping tools (for mockups creation as well as live feature navigation)

PS2. Also make sure to look at the MS tool (Sketchup in MS Expression Studio):
https://msdn.microsoft.com/en-us/library/ee341458(v=expression.40).aspx

https://msdn.microsoft.com/en-us/library/ee341449(v=expression.40).aspx

PS. Kudos to Vera Moreira for pointing this one out.