The de facto source for basic game info. Bookmark it - you will use it, and often.
Colosseum Web App
An amazing web app that will tell you the best equipment for questing, raiding, leveling, pvp, and colo. Takes a little bit to first enter your info (although there are some shortcuts), but it makes a massive difference usually. Highly recommended.
The official forums for the game.
DotD Log Analyzer
Wonderful script that will make your hit logs readable and sortable. Fooby logs are a great way to pass along data as well.
Damage Procs Guide
The definitive guide to how much damage procs do and how often they occur.
Magics Full Report
Forum thread with a list of the magics, their rates, and damages.
Generals and Legion Stats
A great sheet giving info on all the generals and legions, and also the PVP bonus from each.
The updated version of the old script, now using a database of raids.
Yepo's Raid Catcher Add on
Plug in for the Raid Catcher script that has a mini version of the tnl calculator, special sharing tools, land calculator, and much more.