Addons that help with leveling.

Here are some addons I use to help with leveling.

Tour Guide
TourGuide is an ingame leveling guide I've used the paid guides and this free guide is better than all of them.

Doublewide
DoubleWide is a simple AddOn for a simple idea -- pull the quest log into side-by-side areas so it's double wide instead of cramping the size of the quest list.

Light Headed
LightHeaded is a very simple addon that displays quest information and comments from http://www.wowhead.com in game, eliminating the need to Alt-Tab when you get stuck on quest.

TomTom
TomTom is your personal navigation assistant in World of Warcraft.

Cartographer
Cartographer is a modular, lightweight, and efficient framework for manipulation of the world map.
Get the latest Cartographer here.

QuestHelper
QuestHelper compiles a list of waypoints for the quests in your quest log, and sorts them so as to minimize the distance between them, and so that they happen in the correct order.
Make sure you enable the tracked filter or it will interfere with TourGuide.
Just a heads up QuestHelper has a known bug where it will lock WoW up when starting a timed quest.

There is a FAQ about installing addons.

Enjoy