Zygors Leveling Guide (a Downloadable Addon) is a well known World of Warcraft leveling utility. It has been around for a few years now, evolving from an e-book to a fully functional In-Game Leveling Addon.
The addon has improved over time and is always updating content with each patch of the game. I have only used the Horde Leveling Addon. So any of the Alliance Leveling Addon I can not vouch for.
I have used all of the other well known guides that are web based. They all were well done in their own right and I enjoyed them. The major difference of course being that this guide was an addon. When it first came into the game it was not as user friendly as it is now. Many glitches and annoying bugs have been ironed out.
So what does it do?
Well once you use the installation wizard and get it all in there. You go in game and now have a little movable panel on your screen. You choose which guide you want to use (depending on what level you are at) and get going. There will be an arrow pointing the way and also points of interest marked on your Map and minimap. In the panel there is a short but explanatory of whatever part you are on and what you need to do next. As you progress the guide will automatically move from step to step as you do.
That's the basics. It's hard for me to say just HOW LONG it takes for you to get to 80 as most of the time I stopped in between to level a profession or idle for whatever Reason and Played every day so I did not have much rest exp. So results vary on that. However I was able to go through most of the game without grinding at all.