Below I've compiled a list of addons/scripts that I highly recommend you install to have a more successful end-game experience. A good majority of us are using these (I hope), and we want you to benefit from them also. This is not an exhaustive list by any means, but a few key ones that will benefit all classes.

1. Cosmos - http://www.cosmosui.org/index.php?p=download (probably the most useful addon in WoW if you don't yet have it).
2. CT_RaidAssist - http://www.curse-gaming.com/mod.php?addid=694
3. CT_RABossMods - http://www.curse-gaming.com/mod.php?addid=3186
4. CT_RaidTracker - http://www.curse-gaming.com/mod.php?addid=1287
5. QuickMountEquip - http://www.curse-gaming.com/mod.php?addid=282 - see below if you want simple macro (less functionality) instead of another addon! :P

Here's a macro I wrote a long time ago that some ppl were asking me for, that will automatically equip your Carrot on a Stick, Mithril Spurs and Glove Mount Speed Enchant, saving yourself from another addon. You must always keep these 3 items in the same bag slots however, as the function UseContainerItem(x,y) pulls from the defined bag slot. Also remember that if you get dismount (when running inside for example), you must click the mount button again (you can keep running), in order to swap in the correct gear. Create a new macro and copy the following script into it:
/em Come here you lazy mount!
/script UseContainerItem(x,y)
/script UseContainerItem(x,y)
/script UseContainerItem(x,y)
/script UseContainerItem(x,y)
Usage of UseContainerItem(x = bag# from 0-4, y = slot number 1-x)
The first function parameters must contain your mount.

There are several others that would also be very useful including AutoPotion, CQManaConserver, etc. but the above ones are probably the most popular.

Cheers!
Eganwp