Official Guild Wars 2 Wiki: The title pretty much sums it up.
GW2DB: A complete database of all items in Guild Wars 2. Looking for info about an object? Look it up. Want to know how to get it? Read the comments.
Orrmaps: Up to date information on the location of valuable resource nodes. It will save you some serious time farming.
GW2 Gold Guide: No, it's not for buying gold; it's filled with tips and tricks to help you level and farm the materials that are hot at the Trading Post
Mastodonte: A tool for calculationg combos.
GW2 Builds: Try the Build Calculator; browse pre-made builds and see what people are saying about them.