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.
Quest Unlocked Guides
This is a wiki type resource for DotD. Really handy land purchasing calculator, and a nice reference for other data.
Fleecemaster's Raid Farming Guide
Fleece has come up with a fantastic guide to effective raid farming, and has an intriguing new optimal damage number he has coined "Real Share". Definitely worth a look.
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.
For The Good of All
Fork of the older script, so it allows for pastebins, etc.