Balsamiq Mockups is an incredible application for creating user interface mockups. The application allows you layout interface sketches using the 50 interface controls included. Controls are added to the canvas or notebook by dragging and dropping or by using an insert input with auto-completion suggestions. As with Visio, many controls may be easily customized by editing values in the properties panel, and may be modified to show state, e.g. selected, disabled, etc.
This looks like a very interesting application for sharing wireframes and mockups via one of the CMSes that provide support for it. It does not have typical drawing tools, so you cannot customize the stencils at this point, but this should serve very well for many people's needs around communicating interface ideas.
The application is built on Flex and is available as a client plugin for systems including Confluence and Twiki, and may also be run as a standalone Adobe AIR application. It allows export to XML for importing into new mockups, or to PNG.