Inspiration and Kidspiration have been around for a long time. The cool thing about them is that they are designed for educational use. The iPad apps for each one are really fun and easy for kids to use. There is a great deal of online content that can be accessed, also.
Inspiration Maps Lite is free in the Apps Store. The icon looks like this:
It's easy for kids to manipulate their word clouds or mind maps by just tapping and moving them around. They can also add connectors easily.
Kidsipiration also has a free version in the App Store. It's great for younger kids (grades k-3) and provides great graphics and ease of use.
The icon looks like this:
The cool thing about Kidspiration is that the students use the correct vocabulary for the types of maps they create.