18-28k permanent intellect buff as a Mage !

This trick revolves around one item : Black Blood of Y'Shaarj. It's a caster trinket that drops on Garrosh. I only tested with the Normal, non-warforged version, but I assume it works with every version of it.

A bit of technical information here : When this trinket procs, it applies two buffs on you. First : theWrath buff, which is a 1 to 10 stack counter. This buff also applies the Intellect increase to your character. Second, the Wrath of the Darkspear buff, which is a countdown from 10. When Wrath of the Darkspear falls off in any way (expiration, trinket removal, buff removal) the Wrath buff falls off too.

tl;dr : The Wrath buff increases your Intellect, and is controlled by Wrath of the Darkspear.

You guessed it, all we have to do is keep the Wrath buff alone and get rid of Wrath of the Darkspear.

There's the "Mage only" part. Equip the trinket and make it proc (dps a training dummy f.x.). Now wait until it reaches 10 stacks. When Wrath of the Darkspeak is about to timeout, use Alter Time. Duringthe 6s of Alter Time, you must get a second proc. It's completely random, and it took myself around 50 tries to reproduce it (I did this first by pure random luck in a duel). And if you're lucky, you'll get theWrath bug alone, fully stacked and never falling off.

But wait ! There's more !

You can remove the trinket, log out, zone out and enter instances and keep the buff ! yeah I promised you big stuff 

Be careful, death cancels the effect.