Hey guys,
I also found a few addons that are invaluable to me now and I can't imagine what I did before them:
Coordinates / Mapster - Links in to Lightheaded
Auctioneer - Helps you value items you pick up and make for sale on AH
Omen - threat meter...very valuable for the bigger instance runs!
Lightheaded - Directly links you to WoWhead in game so no more [ALT][TAB] to check the internet for guidance on a quest and gives you co-ords of 99% of the quests on offer.
All of these can be downloaded from www.wowinterface.com
(I use a LOT more than those 4, but these are must haves in my opinion!)