The Visual Understanding Environment (VUE) is an Open Source project based at Tufts University. The software seems quite mature (current release v3.1.1) and strikes me as a crossover of a graph visualisation/layout and a mind-mapping tool, resulting in a different way of sharing and presenting information – think prezi.
VUE, the Visual Understanding Environment, is an open source concept mapping tool, licensed under the Education Commons License. Although originally developed with a focus on educational needs, VUE has grown to be used by people in many organizations working on engineering, graphic design, creative problem solving, construction, writing, and more. VUE 2 introduced the melding of the map with presentation tools, allowing people to advance from the linear nature of traditional slide paradigms to a more flexible model allowing presenters to walk their maps in dynamic and visually controlled manner.
VUE is available for all major OSs – the videos below give an overview of the capabilities:
Check out the VUE 3 User Guide or the VUE Webinar for more in-depth info. Friends of the dead-tree format might interest that there is even a softcover book about VUE available (found via SchockWellenReiter). It might be suitable for a few projects, but I am also considering neoClipse due to its seamless integration with a graph-database.