Here's the one I've been using since 2.4 came out. Inspired by many different UIs I've seen, unit frames inspired by Ryas/Nitelily and many more. This UI is built entirely from scratch - I like to imitate work I've seen, helps me get to grips with the add ons and understand how people have come up with such fantastic looking UIs.
Idling in Shattrath: The panels surrounding my player frame and the black panels below are eePanels2, which shows on the same opacity as the player frame, here 50%.
Completely empty: When I'm not targetting anything, and also is out of combat + 100% HP and Mana, my unitframes completely disappear. I also have a macro to hide my actionbar, which is Bartender3, for the record.
In combat, with party: Target's target shows here. This frame only shows in combat. Also, you can see a frame on the side. That's the party frame. The party member frames scale downwards. To the left of the target's target frame, you can also see the focus frame, currently targetting a Royal Teromoth.