As the title reads, I'm looking for open source alternatives to balsamiq mockup for prototyping. Anyone knows of an equally good alternative that's open source or basically freeware.
The best available Open source mock up tool is Pencil. Its available as firefox plugin as well as stand alone.
Another solution, which I personally use is Inkscape, an open source SVG Editor. It is NOT a mock up designer, but we can use it for designing mock ups, using freely available stencil kit, like Yahoo Stencil Kit.