This radial browser was designed to display complex concept network structures in a snappy and intuitive manner. It can be used to visualize conceptual structures, social networks, or anything else that can be expressed in nodes and links.
You can download a preliminary Open Source version of the software here:Relation browser source code
Note that the code is already a couple of years old, so it is programmed in Actionscript 1. But given that, it is quite solid and usually works well. The code is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike License. If your project serves a non-commercial purpose, the use of the browser or a modified version is free, if you leave the copyright note intact. Please contact me, if you intend commercial usage.
You might also want to take a look at