I’m following various free & open source prototyping tools over the years but always found them a bit more clunky than pen & paper or marker on a board. How about you?
Here’s a list of the ones I found. Do you us any of these tools for your work? Let’s discuss! 
(I also started a poll on Twitter)
I use non of these (except pen and paper).
They all seem to have interesting concepts, but are either on halt or very early in development. Even a tool like Alva (which seems to be among the more mature) quickly crashed when trying it out. I have the feeling that such a tool would need to be backed by an institution or donating community to get tools beyond being half finished. (I myself have created such a half finished tool, see https://jdittrich.github.io/quickMockup/ )
Alas, at work I use figma, which works well (but is not open source).
I recently made a full website redesign using only F/L/OSS tools and tested pencil, wich I didn’t knew at the time and Inkscape, wich I am a long ter user but only for printing/illustration porpouses.
I found Pencil quite clumsy and there were a lot of small papercuts with simple things like alignment, element seelcting, font-size, layers… Also missed some easier way to use custom graphics, I did a quick research about, saw that it was no so straight-forward and gave up.
Inkscape surbprised me when making mockups, by working with svgs I could open the files in the browser and link them using javascript into the inkscape interface. And being a vector-graphical software I was able to build whatever components I wish.
For user-testing porpouses I preferred Inkscape because I could show the protoype with a more refined look.
Links to the Pencil screens and Inkscape Screens. (both in .png)
I’d never even heard of most of those. Presentator looks like a decent alternative to InVision though so I might give that a try. Besides pen and paper I mostly use proprietary products for my design work.
I mostly use invision. It is good in every aspect. Playing around with invision is a great way to start wireframing, prototyping and creating mockups…!
Pencil development is not halted: https://github.com/evolus/pencil/tree/development
Nightly builds from the development tree: https://pencil.evolus.vn/Nightly.html
I use pen & paper, though
No, just pen & paper
Other – let’s discuss!