App of the Week hasn't gone so well so I have switched to App of the Month in the hope I can manage that. If not there is always App of the Year or even Decade.
This month it is XMind. I have been using this extensively for study in the last few weeks and have found it very useful.
It has a very clean, crisp interface. A great set of keybindings and the stuff it produces is very shiny.

It is available on Windows, OS X and Linux.