This addon is a must-get if you’re planning on doing some interface customization and you want to align your bars and frames perfectly!

http://wow.curse.com/downloads/wow-addons/details/align.aspx

This addon is a must get if you’re an achievement addict.. Er, I mean if you’re planning on doing some achievements.

http://wow.curse.com/downloads/wow-addons/details/overachiever.aspx

Altoholic is really awesome! This addon shares all sorts of data between your characters and guildmates!

http://wow.curse.com/downloads/wow-addons/details/altoholic.aspx

GrimReaper is an addon of pure evil!
.. Well, not really. GrimReaper is really useful if you want to see why someone in your raid group is taking up so much damage.

And no, you don’t need to perform any rituals to get this addon!
http://wow.curse.com/downloads/wow-addons/details/grim-reaper.aspx

This addon actually doesn’t have anything to with ready checks! Instead it shows you the gear rating of your party or raid and the addon tells you what instances your party is capable of doing.

http://wow.curse.com/downloads/wow-addons/details/raidreadycheck.aspx

This addon is a must-get if you need to constantly keep your eyes on your abilities cooldowns!

http://wow.curse.com/downloads/wow-addons/details/coolline-cooldowns.aspx

This addon is a must-get if you want to know how much your spells and abilities hit and crit or if you want to do some damage testing!

Finding rare mobs is getting a lot easier! _NPCScan finds rare mobs for you without the need of any macro spamming!

A must-have if your planning on doing achievements like Bloody Rare and Frostbitten!
http://wow.curse.com/downloads/wow-addons/details/npcscan.aspx
I also strongly recommend using Rare Spawn Overlay when using this addon.

WoW players probably have many different addons for different situations. With this addon you can easily switch between different addon profiles.

I wonder how many times I have jumped since I started playing WoW?

http://wow.curse.com/downloads/wow-addons/details/jump-counter.aspx