What I like best about Pencil (apart from the fact that it is free, both as in "free beer" as well as in "free speech") is the large set of available stencils. You have sketchy stencils for early wireframes, but also native UI stencils that are drawn using your system's widget theme, as well as Android and iOS controls, common web elements, flowchart elements and more. The GUI stencils are not simply "dumb" vector graphics, but have properties like disabled, default button etc. Wireframes for different UI states can be interconnected using hyperlinked UI elements to create simple "interactive" mockups and export them as PDFs or HTML documents. You can create your own "dumb" stencils just by combining and editing existing stencils and then adding the result to your personal collection, or you can create stencils with all the bells and whistles by writing them in XML.

I like that it’s free? It’s cool that I can download it without any cost.

It's free and open source, and also pretty much the only half decent prototyping tool on Linux.