Searched for this, didn't find anything, hopefully it's not a repost.
As many mages already know, there are some really nice NPC-cast buffs out there, some of which are just downright OP. One such buff is called Arcane Focus (Arcane Focus - Spell - World of Warcraft) and is cast by Focus Wizard (Focus Wizard - NPC - World of Warcraft).
To put it in perspective, this buff gives the same amount of haste as Heroism/Bloodlust and lasts 3x as long.
"How does this benefit me in a practical situation?" you might be asking. Well, on any given fight where you need a lot of dps in a short period of time, this buff would certainly come in handy. Obviously you'll need a warlock to summon you in immediately prior to the pull. Assuming that it takes 40 seconds from the time you first push the Spellsteal button to the time you can start freely casting on an enemy after being summoned, that still leaves you with 80 seconds of +30% haste, twice as long as Hero/BL.
Considering the value of haste to any given mage, this is a significant boost to ones own dps as well as the overall damage output of the entire raid. Depending on how many mages you bring along, this has the potential to be the difference between success or failure on a new hardmode.
Hopefully someone will be able to use this to progress a bit further in their raids.
NOTES
I realize that if a wipe occurs, it's very time consuming to go back to the location of the NPCs that use this buff and spellsteal it again. This is not for first-attempt encounters that you will most likely wipe on, this is for encounters that you know you can defeat with that little extra push.
There is another version of Arcane Focus by the same name, cast by Spectral Teachers in Scholomance. This version grants 66% haste but only lasts 30 seconds, making it VERY unlikely that anyone will get use out of it. Do not confuse the two!
There is a buff called Zeal cast by cultists north of the area where the Focus Wizards are. It grants 20% haste, lasts the full 2 minutes, and acts as a fine substitute if you somehow can't make better use of Arcane Focus.
I'm aware that this general trick is probably common sense to some.