mindmeister.com
Online Mind Mapping: MindMeister - free web-based collaborative mind mapping tool for brainstorming and project management
nelements.org
the official site of Nelements - a 3d knowledge representation system, a 3d Mind mapping tool, and a 3d Knowledge Organization Tool. It is also a 3d concept mapping tool. You can use Nelements as a 3d outliner to outline your ideas as well.