But after I've already started playing, I typically don't like to bother with external programs and websites. To illustrate, consider the following hypothetical situation: you're in the middle of a quest you thought would be simple and those Disgusting Whosamawidgets simply refuse to drop the required Sticky Yellow Fluid. You've been wandering around for an hour killing (and getting killed by) these horrid creatures, and you feel stopping to open up your web browser and check Wowhead would only add to your frustration. Finally you call out in desperate all-caps leetspeak on the general channel, only to be thoroughly ignored! Then finally one wise and tender-hearted player informs you that were actually supposed to be killing the Grotesque Whosamawidgets just next door, not the Disgusting ones. You thank her, of course, but in your gut you're still feeling dumb and regretting having wasted so much time and energy.
It's enough to make you feel lightheaded indeed! Thankfully that's where Cladhaire comes to the rescue with his addon designed to bring those helpful sorts of people and their comments on Wowhead right into the game for you. Lightheaded (the addon) opens up just to the right of your questlog and shows whatever wowhead's commenters have to say about the quest you click on. It only loads that portion of the quest comment database your quest is in, too. This helps to save on memory usage, save on trips to a website, and also save on massive frustration all in one lovely addon.
And... on a related note, consider using DoubleWide (shown above, working with Lightheaded) to make the questlog into two panels instead of one. This makes quests and quest lists so much easier to read, Blizzard really should incorporate it into the standard interface.