If you want your students to do visual brainstorming and/or to create outlines for writing, MindMup2.0 is a great mind mapping tool that’s free, but it does not have the ‘flip’ feature (flipping from a map to formatted text and back again) that Inspiration has. There is also another tool named Coggle that is easy to use.
Text 2 Mindmap is another free tool. I wouldn’t have thought it would be very intuitive, as you have to create your mindmaps in text form using tabbed lists, but that is an actual technique that middle school students are taught, so it can also be an option.