Zygor's 1-80 Alliance & Horde Leveling Guide
This is without a doubt the best leveling guide available! I've used it myself and highly recommend it. It features an in-game addon to help you along on each quest. It also gives you the order of quests to do, the location of which quest to do next, and automatically progresses as you complete the quest. It even shows you on the in-game map where to go and what to do. It is a totally in-game guide. Gone are the days of switching windows to read a leveling guide!Click Here To Visit Zygor's Site
Here is a screenshot from my personal use of Zygor's Guide and a link to the toon's WoW Armory page. (I started using this guide when this toon was Level 24 and flew to Level 70. Using it from Level 1 will make your played time much much lower than mine.)
Armory Page of my toon on Stormrage Server: Hoofman
Click the image to see the full screenshot. 
Zygor's Announcement of Zygor's Guides 2.0
Brian Kopp's 1-70 Alliance Leveling Guide
Brian Kopp's Original Leveling Guide! Its a great tool for any alliance player.
Team iDemise Alliance Leveling Guide
Recently updated to 1-80, this guide is growing in popularity by leaps and bounds. This leveling guide also features an in-game interface. You never have to alt tab out of the game to look at a guide.
