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This is a great application for a wiki!

Any wiki will do, but I especially recommend one in which you can offer logins for your players so that you can classify information for yourself, for the players en masse, and for each player individually.

The categorization features of a wiki are important, but not as important as good full-text search.

I like Dokuwiki myself. I put different versions of maps up for my own reference and for players. NPC lists, economic notes, geopolitical features... If you play the sort of game where players journal their accomplishments, it's a good place for them to store that.