Sites that have really done their homework and figured things out about the wonderful world of Tyria. Dulfy puts out awesome articles and guides for all of the content around Guild Wars 2.  Here are the ones I find myself referencing all the time:

World Bosses: Tracks which boss is coming up and you can use it to track which ones you have finished for the day.

Ever wonder where to find specific resources? Shows where all the farms and permanent nodes reside.