By popular demand, the link I used to access the wonderful wonderful addon Quest Helper.
I've read that some people don't actually like the addon, because it makes questing too easy, and takes some of the zest of exploration and challenge out.
Personally, I love it. When I first started playing WoW, I didn't know about WoWHead or thottbot, or the likes. I found the nature of what I came to call "blind" questing to be hugely frustrating. Its not like you're trying to solve puzzles or creatively tackle problems. No. You've got to find the one spot in this giant world where the one type of mob among thousands drops the one item you want.
If the only "challenge" to a quest is to blindly kill random stuff until you find the mobs that drop what you want, then that kinda bores me. Its bad enough that once you find the mobs that drop what you want, you still have to grind through piles of them to get your items. So I research the quest on WoW Head, or Thottbot, to remove part of the drudgery.
When I first found those research sites, it was a giant improvement to my play time, especially because I've got 2 computers with monitors next to eachother, making access quite natural and easy. But even when one is shut down and I Alt-TAB to WoWHead, its still better than the in-game quest text.
QH takes it one step further, and integrates the experience.
And the benefit to parties? HUGE. How cool is it to be able to see shared quest goals so your party can really go help each other clear out those pesky group quests that're just laying there in your log for ages and ages.
Now if it could add a feature that will assist you in bringing a party member up through a quest chain to get sync'ed with the other members? That would be priceless. How often have you been in a group, ask for "share pls", only to find that you're unqualified?
If they could help you get caught up, w00t on them. Those types of features are, IMHO, the exact type of stuff Blizzard should be including by default to encourage socialization and group activity. Sure, sometimes you just want to solo and get away from it all, but if they made it that easy to cooperate with one another, the game would be so much cooler.