Duong Thanh An has begun work on Pencil, an open source diagramming tool to free users from commercial products such as Visio and OmniGraffle. Pencil is an application that runs on the Mozilla Gecko engine and can be run as a Firefox 3 tool, or in standalone mode (Windows and Linux only for now).
The first version looks very promising and provides the basic functionalities needed to create diagrams and page schematics. What looks most impressive to me presently are the features of the Windows UI stencils that I've missed since moving away from Visio. UI objects in the Windows XP Widgets stencil provide properties for changing the state of a control, e.g. right click a form checkbox to select disabled or selected, and the control takes on that new state. Right click a combo box and select disabled or focussed. This is one of the functionalities in Visio that just never made it into OmniGraffle.
Pencil also provides canvas masters using the background page model Visio introduced, which I find to be pretty well implemented. There are still a lot of features that need to be implemented for me to consider using Pencil as a replacement tool, however. Basic shape and line drawing tools are absent, for instance. And while PNG exporting is good, I would need support for better export options. PDF or even SWF of entire documents would be ideal. For a first release, this looks very promising.