Pencil Project is a new prototyping tool that’s a plug-in for Firefox 3. Since it’s browser-based, it’s cross-platform, and it’s open source and completely free. Unfortunately it doesn’t have any interactive abilities (yet). I haven’t had a chance to play with it yet, and I doubt it will replace tools like Visio, but it looks like it will be a good affordable tool for wireframing and prototyping.