This probably isnt going to be anything new to most people, but none the less - heres some sites I recommend taking a look at:
If you heavily into endgame content – I have been introduced to a couple of fantastic equipment check sites. These sites basically check your character in the Armory and make checks against requirements for various end game raid instances or heroics, and tell you where you can expect to work, where upgrades can be found, and any areas of twinking your character has not received (e.g enchants, mismatched gems etc.)
The first of these is www.be.imba.hu – I have used this one to check various characters – both my own, and other peoples.
The second of these sites is http://www.wow-heroes.com (Found this one on posted up on Big Bear Butt blogger’s blog.) I have not personally checked this site yet (I am trusting him this much :p)
On a separate note – if you are interested in playing with the new WOTLK talent trees, they can be found at http://www.wowhead.com