OPie has revolutionized my UI in a way that absolutely fits with my clutter-free mindset. Ultimately, as I test addons, only a few end up being a permanent part of my gameplay. I can't necessarily promise OPie will stay, but it's looking like such an innovative mod that I may just come to rely on it.
We've discussed both Totemus and Necrosis here at Addon Spotlight, and praised the way they wrap things into a neat little package. OPie does the same thing, but to an extent that not only are your spells hidden until you need them, but they can be easily integrated with a minimalist play-style.
What OPie does is take spells, items, trade-skills and macros and creates virtual rings for them to reside in. By assigning a keybinding, one can simply press a button and have a quick menu (a ring) appear on the screen, which will cast a spell, use an item or a macro based on where the mouse is hovering in relation to the ring.