Have You Tried bubbl.us! ??

bubble.us is a simple and free web application for brainstorming.

To start brainstorming, click inside a bubble and type, or

• Press ENTER key to add a bubble below, or
• Press TAB to add bubbles next to the one selected.

The help area on the right hand side helps you as you go along.  You can save, print and export your sheets.