The original project is abandoned, but a recently updated fork is available at github.com/prikhi/pencil.
I prefer Pencil to more commonly referenced options like Balsamiq. Looks like no commits in a few months, but hopefully the form is still going. An open source UX tool focused on software application workflow prototyping rather than GUI layout is an asset to the software development community.
Looks like the original creators rebooted it under the github project https://github.com/evolus/pencil.