"mind mapping" apps?

Does anyone have a favorite mind mapping app for a PDA or iphone?

I am experimenting today with a basic free iphone app. I've wanted to use a digital one for some time to explore sundry practical and slightly ephemeral territories.

Here is my first exploration starting from the central word "Simple", as in 'Simplify". This is not a comprehensive or properly taxonomized map of life - this is what came out of the convolutions of both my more educated thinking and a realer expression of interior, how apparently I am organizing at this moment. It's a start for today.