Mind maps are useful tools for students to summarise what they have learnt. It could also be a brainstorming tool. You There are a number of free online mind maps your students could use. You can export the mind maps as images.
1. https://bubbl.us/
This is pretty colourful. It is free for up to 3 mind maps.
Check out https://bubbl.us/examples for more examples.
2. http://mind42.com/
This appears more linear. You can add images but you can only provide an url. You can add icons too. You can work collaboratively by adding email addresses.
3. http://mindmapfree.com/
Quick and easy without signing up for an account. No images. You can change the colours of the nodes. Very basic but looks more like a mind map in my opinion.
You can check out this mindmap for a comprehensive list of ICT tools.
What other free online mindmap tools do you use?