I've been using Popplets as a simple mind mapping/whiteboard tool which works well for what I need. Videos on youtube.
I used to use iThoughtsHD but ........
... one of the comments above was why aren't more people using mind mapping. I've been mind mapping for several years now and I think I can answer that. The issue is this; many problems are just to complex, with too many dependencies for a mind mapping process.
A mind map is very hierarchical, and many problems are not. While a problem can be broken up into a hierarchy of sub tasks/problems, there still remains many dotted line dependencies. These are impossible to note with many mind mapping applications.
However, something like Popplet gets around that. Popplet is free form, so you can create a traditional mind map, but also add in the dotted line dependencies (or at least you could if Popplet had dotted lines - still in development I believe).
The lack of development is a drawback with Popplet. It has the potential to be a killer mind mapping app, but right now it is missing some features. (line properties being the major missing feature).