Bubbl.us is a Web 2.0 tool that enables users to create mind mapping and brainstorming diagrams online. To begin, the main topic or concept is entered into the parent bubble. Then ideas and thoughts are recorded in colorful text bubbles linked to the parent bubble. Users can continue to add text bubbles which are color coded according to hierarchy. The site is simple and easy to use. The application is Flash-based so the latest Flash viewer needs to be installed. Source: 
Bubbl.us is a (free) Brainstorming service that allows several users to work together in groups if they are users on the site and you give them sharing rights. You can share your work both as HTML or via a read-only link. Bubbl.us is very easy to get started with, so we can recommend it even if you are new to brainstorming activities.
Benefits of using bubbl.us:
- Easy to use and permits graphic representations.
- Does not require an account unless you want to save work
- Helps to organize thoughts and explore relationships
- Aids in generating ideas,
- Drawing pictures or pictographs, Also can be used for non-linguistic representations.
- Encourages risk taking
- Encourages group discussions
- Incorporates multiple intelligences
- Mind mapping is a good tool for visual and kinesthetic (tactile) learners. Visual learners benefit from associating ideas and concepts with images.