It could be Spell Accuracy from the Players Handbook. That allows you to omit a number of squares from your area or close powers up to your wisdom modifier. That feat is epic tier.
At my local gaming store, me and our group invoker had a very interesting discussion. We were "oooh"ing and "ahhh"ing at her sweet sweet minor-action blast spell. Unfortunately, she hit an ally with said spell.
She later told me that she was jealous of my wizard because of a feat that would make blast spells not hurt allies. I totally want in on that action!
She didn't know the details of where she found said feat. I'm not sure if it's heroic, paragon, or epic, or even what book its in (if it exists)! If anyone knows the details, please let me know!
Sweet, thanks a bunch!
You might also get some use from War Wizardry, its Paragon tier and gives you a -5 penalty to hit allies and allies only take half damage.