Official Sites:

Turbine Entertainment - Official Turbine site
Turbine ACDM Site - Official AC site, go here for current news and information from the developers

Asheron's Call General:

Crossroads of Dereth - Good site for up-to-date AC news. Large library of information about the game
ACVault - Provides news and information about the game and a very popular forum for the game.
Maggie the Jackcat - Contains a wealth of information about the game, great place for information on patches and quests.
ACLore - Good quest information
AC Maps - Great site to find maps for dungeons and locations in the game
AC Stratics - General AC news and information site.
AC Springcorner - Great quest information and walkthroughs
Quest Timers - List of quest timers for various items
Auberean Crusaders - Quest information and steps
Player Photos - Pictures of AC players
HG VNboard - Harvestgain forum on ACVault

Third Party Software:

ACDualClient - Great utility for logging in accounts
ACExplorer - Great utility for finding places and routes
Third Party Paradise - Contains a list of many third party software

Decal:

Official Decal Site - Official Decal site
Sucamarto's Decal Site - Great resource for decal plugins and when updates are released for them

Helpful Links:

Bandit Sight Loot Statistics
Caerthalion's Tinkering Calculator
Caeryn Dryad's Weapon Damage Calculator
AC Character Builder
Hiro Yamada's Melee Defense Calculator
Natural Resistance and Regeneration Calculator

Special Thanks:

grsites.com