bubbl.us in their 2009 Top 25 Web sites for Teaching and Learning.
Click on the Start Here button in the center of the next screen. At that time two small squares appear on the right and bottom of the bubble. When you click on the right square another bubble is generated. When you click on the bottom square a "child" bubble is created for that bubble. Click on the Start Here text to add your own text.
A cool feature is that bubbles can be clicked and dragged on the screen for placement and the screen itself can be clicked and dragged for viewing and working purposes. Bubbles can be collapsed (hidden).
An extensive Help section explains very well other features of this simple to navigate, but very useful application. One feature that is easy to accomplish is the labeling of lines connecting bubbles. Just click on the lines and add your text.
Here is an example of a sheet with a variety of bubbles that I was able to generate very quickly. I can see using this application almost any time a class wishes to discuss a variety of topics. We are reading The One and Only Ivan by Katherine Applegate as a class read with our fourth graders. I will be using and expanding this sheet.