Possibly the most complicated Addon I have ever used it is also one of the most powerful. You can build custom Visual and Audio warnings for virtually any situation in the Game. I have used it for Buff Stacks Low HP and Ability Cooldowns many times over the years on various Characters. Blizzard added similar Visual Warnings in Cataclysm.Git Repository: https://github.com/Geigerkind/ModifiedPowerAuras
I also wanted to note that a lot of users, I found on the Web claimed Dugi’s Guide to be faster.  However, all these users had either used Zygor Guides for very little time or had not used it at all.  I have to give a point to each Dugi’s Guide and Zygor Guides as they both deliver a great lvling speed.  For all you Dugi’s guide users out there who really feel that Dugi’s Guide is faster, I will give Dugi an extra  point.  If there really were a difference, it wouldn’t be by much.