In all the guilds travels you run across some really cool sites that help you out. We decided to put as many as we could together here to help everyone out, enjoy...

Gear Audit: great site for finding out where your toon is best suited for and an overall evaluation of you gemming and enchanting.
-- http://be.imba.hu
-- http://www.wow-heroes.com/

Gems: wowwiki.com is great for many things. I use it to look up loot tables and much more. This link is for gems, great overview of all gems in a simple table.
-- http://www.wowwiki.com/Gems

Gear:
-- http://www.wow-loot.com
-- http://www.maxdps.com/

Addons:
-- http://www.curse.com
-- http://www.atlasmod.com/
-- http://auctioneeraddon.com/
-- http://www.wowinterface.com/find.php
-- http://www.ctmod.net/downloads

General/Miscellaneous
-- http://www.thottbot.com
-- http://www.wowhead.com

Character
Great site for various classes. Breakdown of stats how they apply, couple of spreadsheet, nice site. I get all my Pally information from here:
-- All toons - http://www.elitistjerks.com/f31/
http://www.manaflask.com/
-- Rogues - http://www.shadowpanther.net
-- Hunters - http://www.femaledwarf.com/

Raid Boss Fights:
-- http://www.bosskillers.com/
-- http://www.tankspot.com/

Armory:
-- http://www.wowarmory.com

Arena Points Calculator:
-- http://www.wowarmory.com/arena-calculator.xml

Professions:
-- http://wow.crafterstome.com/home.html
-- fishing - http://www.elsanglin.com
-- Vanity Pets - http://www.warcraftpets.com
-- Hunter Pets - http://petopia.brashendeavors.net

Signatures:
-- http://sigs.guildlaunch.net
-- http://wow-avatar.dyndns.org/EN/create_signature.html
-- http://sigs.azhriaz.com/

Various:
-- http://www.warcraftrealms.com
-- http://www.magelo.com

RAWR, great little application that runs locally, so one time armory update and you aren't waiting for webpage refreshes or anything. Allows you to lookup gear, compare gear, gem items and apply or remove enchants to see how they affect your numbers. Really allows you to play with your paperdoll to see how you numbers change as you change things. Only has support for certain race/spec but they are adding modules for the others.
   http://www.codeplex.com/Rawr/Release/ProjectReleases.aspx?ReleaseId=13446 I do not know if the app has been updated for 3.0 patch.

Im sure there are a bunch more but those are the basics I use. If you have a great link you would like to share post it in the general forum and I will then update this page with it.