Bubbl.us is an online tool for creating mind maps, webs, and other diagrams. It's a nice light little tool that has one job and does it pretty well. You are able to create mind maps using bubbles and connecting lines, both of which have some customization features. You can add text to your bubbles, and customize the text's size, color and font. A nice feature is that you can add pictures to your maps, making it more versatile and something even lower grades can use.
Bubbl.us is also a collaborative tool. If you have a group of students working on a project, they can brainstorm and start a map, share it with others in their group who can then add and edit the maps as well.
I've found Bubbl.us to work reliably, and to be quite stable. The drag to move features are also very smooth, which you can appreciate after using some tools that have jaggy dragging.
Overall it's a nice little tool, that does one job and does it well. If you are working on brainstorming or mind maps with students, it is a great tool to check out.