Built on the Gecko engine, "Pencil" is a Firefox 3 plug-in that has a light, 400 KB installation. Its goal is to be a forever free and open source tool for making diagrams and doing GUI prototyping, yet also easy enough that everyone can use it. For mock-ups, Pencil makes it easy to spin the look you want, then output PNG for record-keeping and distribution:
Since each Pencil document can be saved in a single XML-based file with the .ep extension, I think the diagram/mockup source can be shared as easily as Visio files. No, I haven't tried it yet -- I thought it was interesting enough to share right away. :-)