I was browsing curse-gaming today and found this very cool tool: The Armorer
It was not made by me, so all credit goes to the author, I'm just posting it here since I think it's really great and I think everyone should have it.
What it does is when you press f9, a little window comes up that asks for a players realm and name, it then gets the most important data from armory of that character (hp, spec, resiliance etc).
You don't have to alt tab out of the game so you can fast and easy check who you're up against in arena.
Also, this does not break any of Blizzards rules since it doesn't interfere with the game in any way.
Some screens:
And yeah... I wont post virus scan screens, it's been downloaded many times, check comments at curse if you're paranoid.
I tested it, it works 100 %.