While I am still waiting in the full version, the lite version of popplet is already very promising. You can very easily organize your thoughts using the board and its options of boxes, in which you can insert text, hand drawing, and photos. It is possible to change the color of the boxes and to link boxes. The full version promises multiple boards.
Beyond this, this app needs the ability to group a couple of boxes together to be really outstanding. Right now it see s impossible to select more than one box at the same time. Also, when moving one box, the other boxes that are connected to this box stay at their place. This might be desirable at some times, but it does not allow me to organize my thoughts in hierarchical ways.
In addition, it would be nice if it was possible to include freehand drawings outside of boxes, for example to create conceptual maps more easily.
If future versions show these or similar improvements, it would become a very interesting and powerful app to develop your thoughts.