Bubbl.Us

https://bubbl.us/
Easy to use, quick to get started and create mindmaps

Popplet
http://www.popplet.com/
Great site for visual thinking and development.

Mindmeister

http://www.mindmeister.com/
Mindmeister is a more significant alternative if you’re looking for more developed mind maps in your classroom.

Gliffy

http://www.gliffy.com/
Online diagramming software,to easily create professional-looking flowcharts, diagrams, floorplans, technical drawings, and more.

ThinkLinkr

http://thinklinkr.com/

Organizing information in outlines with the ability to invite collaborators and live chat.

Mindomo

http://www.mindomo.com/

A web-based mind mapping tool. Create, edit mind maps, and share them with your colleagues or your friends.

Exploratree

http://www.exploratree.org.uk/

Free online library of thinking guides and graphic organisers.