Wowhead Quests 

UPDATE: I am no longer supporting this mod. I've started use Cladhaire's Lightheaded and I am really enjoying it. I don't make addons for glory, I simply make them for my own benefit so that the game is more enjoyable. If someone else wants to create a more improved version of what I am already doing, more power to them. And if it's better (like in thise case), then I'll adopt it myself. I hope that's a big enough endorsement of Lightheaded.

Personally, I recommend using Lightheaded with Mobmap. The combination of the two addons together makes alt-tabbing to look something up on the web a thing of the past.

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What is Wowhead Quests?
Wowhead Quests is a UI interface that hooks into the default QuestLog and displays the quest comments from the Wowhead website for any quest that you ALT-CLICK. The comments are stored locally in two "load-on-demand" databases (one for Horde, one for Alliance). Each database is approximately 1.7 megs and only one is loaded at a time.

Why is this useful?
Wowhead is a website similar to Thotbot where users can share comments about specific quests. This typically offer suggestions for completing it and often xx,yy coordinates for where you need to complete the quest objective.

Credits: This is a modification of ostap's addon called qcomment located on Curse. Qcomment is a plugin for nQuestLog and requires it to function. This addon will operate as a stand-alone because it hooks the default QuestLog. It also offers the seperate load-on-demand databases to keep memory usage to a minimum. Also, comment frames will stay open until you close them and you can have several of them open at the same time.