TiddlyWiki - a non-linear personal web notebook
Xmind - Xmind is a brainstorming and mind mapping application.
wikidPad - wikidPad is an application for storing thoughts, ideas, todo lists, contacts, or anything else that user can think of to write down.
MindMeister - Create, share and collaboratively work on mind maps with MindMeister, the leading online mind mapping software. Includes apps for iPhone, iPad and Android.
Knowledge Workshop - Learning Management Solutions provides innovative and reliable software for the knowledge worker.
Coggle - Coggle is a simple, beautiful, powerful way of structuring information.