XMind is an excellent mindmapping tool for this purpose. - Source: Reddit / about 1 month ago
MindMeister - Create, share and collaboratively work on mind maps with MindMeister, the leading online mind mapping software. Includes apps for iPhone, iPad and Android.
MindManager - With MindManager, flexible mind maps promote freeform thinking and quick organization of ideas, so creativity and productivity can live in harmony.
FeatureMap - FeatureMap story mapping, simple and effective realtime collaboration and collective intelligence tool.
Coggle - Coggle is a simple, beautiful, powerful way of structuring information.
Idea Market - Exago is collaborative idea management software that powers the business growth.
FreeMind - FreeMind is a premier free mind-mapping software written in Java.