MindNode is a free Leopard-only mind mapping application from Markus Müller, based in Austria. The application is by no means feature-rich; in fact, it's so simple as to be a bit disarming at first. But what it lacks in features, it makes up for in elegance.
The interface is incredibly clean. To create a new node, simply double-click anywhere on the window, or press the Tab key when focused on a node to create a new child node. To create a node at the same level as the one that is currently in focus, press option-Tab.
As new nodes are added at the same level, they surround the previous nodes. For example, the second new node appears below the original one. The third appears above. Below, above, below, above. While this is an easy way to keep everything grouped together, it's not particularly useful if you're using MindNode to create a list with any sort of ordering. Some would argue that mind maps should not worry about ordering since it should be all about capturing ideas. But given that mind maps are useful in large part because they allow for graphical organization, this behavior is a bit unfortunate.
Other than that one small beef, MindNode is a nice, small, but useful tool that will be right at home in your Applications folder for the next time you find yourself needing to do some brainstorming.