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I'm currently writing a story based in another world. I try to keep the story outline and the world in fairly separate notes obviously dipping into the world notes when I need to remember names, locations, descriptions etc.

Currently I'm using trello because of it's flexibility. I've got lists for Characters, Races, Places, Technology, Religion etc... But I'm finding that it's not scaling very well. A bigger story has hundreds of characters who are all connected, Trello gives me no way to keep track of that (aside from notes on their card).

I have thought about starting a wiki (kind of a fictional wikipedia) because of it's fantastic flexibility and inter connectivity (historical figures link to technology they created and used to their family and their places of birth etc). However most of the ones I've found are open to the public - given I don't want to reveal every little detail about a character's past and future to anyone who stumbles upon it I've tended to rule these out.

What other software is available (open source ideally) to track this sort of web of information (rather than a linear thread in a story) that would be required to document created world?

Edited to expand following this excellent meta suggestion I use Windows and iOS 8.