UI Wireframing Tools

[leeor_net]

Pencil
Forum user havkyp found Pencil, a free open-source GUI prototyping tool that works very well. Some of the wireframes he provided are in this format.

Draw.Io
I've had success with draw.io both for wire framing and for flow charting program logic. It integrates well with Google Drive as well which makes it a very attractive web-based program (no need to download a separate software package).

EDIT:

I attached a Pencil file that can be used as a template.

[Goof]

I do use Inkscape for these needs.
And if I need to extract parts of existing bitmap resources, I use The Gimp. (2.10.0 just released 10 days ago)

[jballou]

Do we have starting documents for things like main game interface so I can add/change things in there?

[leeor_net]

I've been using a template for awhile. I'll attach it to the original post.