What is it and why do I love it? It allows me to devastate whatever my mouse is hovering over. So I just move my mouse over the nameplate of an enemy and hit the mouse-over devastate hotkey. That target gets Devastated and I don't lose my current target. Happy times. It makes multi-mob tanking verrrrry easy.

#show Devastate
/cast [target=mouseover, harm, nodead] [] Devastate

What else can you do with this? Well, you can make it do Sunder Armor instead. Why is that useful? Because if you do something like this:

#show Sunder Armor
/cast [target=mouseover, harm, nodead] [] Sunder Armor

You can use it on sheeped/trapped targets and not break the CC. So lets say you're tanking Fel_Orc_9000 in Ramparts and Fel_Orc_9001 (ITS OVER 9000!) is sheeped next to you. If you mouse-over the sheeped mob's nameplate and use the above macro, you'll sunder it and build threat without actually doing any damage to it and without breaking the CC.

Ok what else do I have? This next one is pretty much only useful in PvP and only useful if you've got Tactical Mastery. (35/23/3)

#showtooltip Spell Reflection
/cast [stance:1/2,equipped:Shields] Spell Reflection; [stance:3] Defensive Stance
/stopmacro [equipped: Shields]
/equip Merciless Gladiator's Shield Wall
/equip King's Defender

Replace MGSW and King's Defender with your weapon and shield. What this macro does is switches you to defensive stance, puts on your sword and shield, and casts spell reflect. If you're in battle stance it doesn't put you in defensive stance since you can spell reflect in battle stance.

Another PvP macro:
#showtooltip Disarm
/cast [stance:2] Disarm; [stance:1,3] Defensive Stance

This one disarms if you're in defensive stance. If you're not in defensive stance it puts you in defensive stance, and pressing it again will disarm.