Bubbl.us is a free, web application that allows you to brainstorm online. It is easy to use and you can use it to create mind maps, outlines of papers and projects, meeting notes, and much more. It's very basic. You just click on the page to add a box, enter information, connect it to another box and continue.
You can use it with or without creating an account. You can save, print and embed your files.
This is another great site to use with students for mind mapping, creating layouts to organize ideas, and much more.
Here's an example from Purdue University:
