Mindomo is another option for mind mapping, concept mapping and outlining

Effortless creation of mind maps,concept maps and outlines to improve your thinking, learning and even your business solutions. Online, desktop, iPad and Android apps are available.

Source: Mind mapping, concept mapping and outlining

Mindomo is an online tool for mind mapping, concept mapping, outlining, and more. The free version allows you to store three maps online. There is a teacher version for $69 a year that allows teachers to have up to 75 students working on Mindomo assignments. This allows students to collaborate and have access to the premium version in their free account. The free teacher version allows only one assignment. They also offer mobile apps, and full screen mode for the maps.