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Thats a part of Bigwigs Bossmods.
It's the built in range indicator on certain phases of bosses.

They are there, but I rarely target anyone. I use mouseover macro'es and grid to heal ppl :)

Update:
Fixed my UI abit.. so made a video of it in-action:...

I have corner indicators for:
Agro, Renew, Prayer of Mending, Magic Dispell, Disease dispell. But on can't use the bottom of the grid for indicators since mana bars are there :P
So i use upper left...

Shouldnt be to hard to replicate..
Left out these addons:
Damn Chat Tabs (remove the tabs from chat like "General" etc)
Grid Mana Bars
GridIndicatorCornerPlus
OmniCC
TitanPanel Mail, Clock,...

Yes. The various data shown at the bottom is Titan Panel. Turned the transparancy down so I don't see the ugly skins :P
Recount show/hide button all the way to the right.

My healing UI as Holy priest.

Solo out of combat:
2769

25-man raid in-combat:
2768

Addons:
Bartender