A whopping 1.2 gig patch to get you ready for the adventure in Northrend is finally done and running. A whole new features and changes have been implemented which will take us some time to learn and adapt. On top of that, the moment you log in to the game you will find out that most of your addons are not going to work =( It happens with a new major patch.
A site like wowinterface.com offers a guide to downloading addons that work with 3.0 patch. And when you head on to curse, they also have some addons guide for 3.0 patch. Or you can find new and updated addons at WowUI.
As of this writing there are some favorite addons which are not working or tuned right yet. Among them is Cartographer ( the replacement for your ingame map and you can make custom notes and more) The altest Cartographer is version v2.2 but it’s only good for showing coordinates at the moment, the waypoints setting is broken. And if you’re a fan of Auctioneer, you have to go with Auctioneer Advance, for the classic is no longer supported nor working with the new patch. The other most commonly used addon is Questhelper, here is a link for the version 0.62 which will work with the patch. (Before downloading, please read the comments from the user which you can find them when you scroll down a little bit).
Good luck in hunting for the new addon fixes 
The new patch (3.03) is scheduled to drop before the expansion hit, so get ready for more headache if not heartache. 
