I got asked to post this as I use this a lot for tanking heroics on my warrior. Its really handy when used in conjunction with [item]Glyph of Devastate[/item] and [item]Glyph of Sunder Armor[/item] tho neither is necessary for its use.
Quote#show Devastate(Rank 5)
/cast [target=mouseover, exists, harm, nodead] Devastate(Rank 5); Devastate(Rank 5)
What this does is whatever your mouse is over, if you hit this macro, it will cast Devastate on that mob. This way you dont need to tab through targets to hit them with it, which is VERY handy in heroics in grabbing some AOE threat in a hurry thats a little more solid than Thunderclap and Cleave will generate on its own. Using all 3 of these together, it will be pretty hard for any moronic DPS to pull adds off you unless they are just being really stupid on adds.
I use a variant of the macro for taunt on my warrior and blind on my rogue. It saves a great deal of time and when executed properly allows for uninterrupted dps/tps.