Bubbl.us is a simple and free web application that lets you brainstorm online.
Because you can:
Click on Start Here to start. The "Help" feature right on the page is very helpful. When you go over a symbol the help feature tells you what it does. Give the students a couple of minutes to play before you actually ask for them to do some mind-mapping.
Once you are ready to save, you need to setup an account. You can share your feature, but you have to share with others that have signed up. The best thing is to set up accounts for students using student1, student2, etc.
You can also share by embedding the code, downloading the mind-map. You can export to HTML or to an xml sheet (HTML coding for web pages). You can also get a link address to send to your friends, but they can only read it. There is also a print feature.
This program is colorful, and does have some feautres to play. If you don't have Inspiration, this is your program for mind-mapping.