Besides the Warlocks Den* (listed on the right side of the page under Favorite Links) there are several guides to the warlock that look pretty thorough. Ten Ton Hammer updated their guide in January of 2009. Besides gear, talents and spells, it includes information about soloing with the warlock through various levels.
http://www.tentonhammer.com/wow/guides/classes/warlock
apbit hosts a warlock guide that seems pretty thorough and which includes a leveling guide and a list of known bugs (always useful). This information doesn't seem to have been updated since the original game came out. There's no leveling information for Outlands, for information. Depend on it at your own risk.
http://apbit.com/classguides/warlock/thewarlockguide.html
I found a site devoted to warlock builds that is updated to Patch 3.1. Definitely post-Ulduar!
http://warlockbuilds.com/
There is a wiki just for warlocks. The link below is to their section on green fire, which is an addon that turns the warlock's flames green. I'm holding out for an addon that allows my warlock to shoot pink rhinestones, but this is pretty cool nevertheless. The wiki has a lot of other good information like guides to raiding or PvP play as a warlock, as well as a list of macros for warlocks. This is a good site to explore. Look at Game Mechanics for a DPS calculator and a list of spell coefficients. I've added this wiki to the list of Favorite Sites on the right side of this page.
http://warlock.wikispaces.com/Green+Fire
Finally, from WOW Head's Class Forums for Warlocks, I found PAWN tag scales for those using PAWN to evaluate gear.
http://www.wowhead.com/?forums&topic=101768&p=1149215
*The Solo MMO Player says the Warlocks Den site doesn't seem to have been updated in a while. Use at your own risk.
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