I'm a big fan of alternate timelines, or stories/backgrounds that are not necessarily cannon, but mesh with the timeline. Webstral's stuff about Ft. Hauchuca is excellent stuff, for instance.
Any kind of user created sourcebook for stuff that isn't handled, or just touched on, in canon is also great - Tegyrus' Czech stuff for 2013 is great for instance.
Modules/adventure ideas/hooks, all of that I love, too.
Basically, anything that gets my imagination going in directions it hasn't gone in before, or places I never considered before is great!
basically, anything
