Native, FOSS GUI prototyping tools?

For reasons I don't begin to understand, its authors have written it as a Firefox plugin, but Pencil is a great FOSS prototyping tool. Like Balsamiq Mockups, it comes with a massive number of prefabricated components that allow you to quickly throw together a demo screen. Also like Balsamiq, you can have multiple pages in a sketch, and link between them, allowing your programmers and designers to "click" certain buttons, and get a feel for what should happen when that occurs.

Unlike Balsamiq, its default mock widget set looks like real widgets, which I get concerned causes programmers to slavishly imitate the mockup they see on screen. Thankfully, a sketch widget template has been distributed with Pencil for awhile now. Also unlike Balsamiq—and hopefully not surprising for a Firefox add-on—Pencil exports its sketches to HTML, not Flash/Flex.

I know Pencil looks weird because it's a Firefox extension, but it actually works very well, and is easy-to-use. I am not ideologically attached to FOSS and don't mind spending $80 for great software, so I'll be honest and say that I use Balsamiq over Pencil. I find Balsamiq's general workflow and ease-of-use superior to Pencil. But Pencil's still a great tool. If you're looking for a FOSS prototyper, I think you'd have a hard time doing better. Launch Firefox and go check it out.

Personally I like doing web-ui prototyping in Inkscape (vector graphic drawing program), it is fairly simple to use and you can reuse most of the mockup when you are going to make the final product.

Try Glade Interface Designer, a GTK/Gnome interface designer, usually used for real applications, but could equally be used for prototypes.