Bubbl.us
Simple online brainstorming application that can be used for concept mapping.
Cmap Tools
Free concept mapping software.
FreeMind
Open-source and free.
Google Docs
Create your concept map using Google Docs.
Mindmeister
The basic version is free. Mobile options are available.
Mindomo
Mind mapping software that can be used for concept maps.
Free demo version. Mobile and desktop options available.
Popplet
Simple brainstorming application that can be used for concept mapping.
Desktop,iPad and iPhone applications.
Mindmup
"Friction free online mind mapping"
VUE
VUE (Visual Understanding Environment) is a concept and content mapping application from Tufts University.