Zim is also my latest fad. I tried it before and then went on to other things, and now I'm back. I use it for taking notes on books I am usually reading with Fbreader. I have a pretty efficient system for switching windows to post the info I have marked in Fbreader into an outline in Zim.

For the moment it is replacing online mind-mapping systems such as www.mindmeister.com -- it works great, but I don't always have a reliable Internet connection, such as when I'm on the train. Then too, sometimes mindmeister is somewhat cumbersome on the N900, though it has vastly improved, but Zim is still easier for my purposes.