OmniGraffle though excellent is unfortunately Mac only (it does export to Visio), but is only a little bit more expensive than Balsamiq. (Visio is another standard tool, but is definitely expensive)
One tool that's easily accessible to almost everyone and can do easy simple prototypes is PowerPoint / Impress / Keynote. (Google Docs is also good for collaboration features, but lacks some of the bells and whistles that make mocking / prototyping faster and easier).
All that said, I'm a huge fan of the sharpies and a scanner route especially in these early stages...
I would be happy to be a community lead if it would be helpful.