What are good Open Source / Free software packages that let you design GUIs quickly, like "on a napkin" style, but also give a bit of polish that you could throw into a slide deck? Preference would be to use some clipart/objects ala Visio, but not required.
NOT:
Maybe not quite what you're after, but the Qt GUI designer is fairly good. I've used it for mock ups before.
Not quite as flexible as a napkin but you can take screenshots when you're done.
Try Pencil which has an addon to firefox. Pretty basic but free (released under GPL).
Balsamiq looks real good http://www.balsamiq.com/products/mockups and I think there's some free or trial version.
There was a thread on slashdot at one point, and this seems to be the best suggestion from there (but it's not going to help create "realistic" UIs):
http://dub.washington.edu:2007/denim/
I have a similar need -- Visio is the best tool I've found, but it is also the only reason I keep a windows VM around, so I would really like to replace it with some OSS (or at least linx-compatible) tool.
I'm looking into DIA / Kivio / Inkscape with the Yahoo UI Stencils, and I'll report back.
Edit: I don't believe this is currently possible with Dia. This page (linked from the walkthrough) describes (some of) the limitations on the dia shape support for SVG. Furthermore, rounded rectangles are out (making it difficult, but not impossible to make "pretty" buttons), and the gradient svg definitions are suspiciously absent from the supported svg tags.
Edit: I've settled (for now) on using Inkscape with the Yahoo UI Stencils (which I've broken out into separate svg files) and a couple little workflow hacks to use the Inkscape open dialog like a custom pallet. Full details, and the broken out YUI stencils are available here: Ciscavate.org
OmniGraffle does this pretty well (along with really impressive flowcharts). OS X only, though.
Depending upon how frequently you'll want to edit the mockups, I might prefer to use pen, paper and a flatbed scanner. Software often just slows me down and gives little benefit.
And yes, I'm serious. :) Most of the mockups I produce are really only sketches used to quickly communicate an idea. As such, I can usually make a new, tweaked one in a matter of seconds on a fresh sheet of paper. A quick scan and, lo, you've got a new PNG to pop into your presentation / source control database / whatever.
For anything more high-fidelity than that, I usually start out with the Forms Designer in Visual Studio to create a basic mockup and then tweak it in something like Paint.NET.
Apologies in advance if this is spammy of me, but Mockery is a new tool for building UI mockups/prototypes that I recently released. It supports the popular sketchy mockup style but also allows you to view your designs as they will appear on Windows and on the Mac.
Microsoft Expression Blend 3 has a tool called SketchFlow that is designed specifically for UI prototyping. You can create pages and controls with a "hand-drawn" look, connect controls and pages so that the prototype actually "runs", and put the prototype on the web so that users can evaluate it, comment and mark it up, and return comments to the development team. It also generates XAML so that if you're building in Silverlight or WPF, you can move code from the prototype to development (or not).
Requires Silverlight for running the prototype on the web. 60-day free evaluation copy available on MS Expression Blend site, along with tutorials.
Is it so hard to understand what "OpenSource" means?? Maybe all commercial-company-affiliates refrain themselves from advertising here.
I really like sketchflow tool too much and used it in my last 2 projects, but also i found this very nice tool: Mockingbird
I didn't test in my projects but it seams very nice.