It's fairly easy to switch between running apps on the Nexus 7 by hitting the Recent apps key in the Navigation bar at the bottom of your screen, but for me—it's just not good enough. I don't particularly like leaving the app I'm currently in to see this menu, and the menu itself is kind of clunky, and overall, not as fast as I'd like it to be.
However, installing a new app switcher solves this problem, and the one I've really been digging lately is called Loopr - Task Switcher, created by Android dev FBarroso, which is a lot like Pie Control, giving you an easily accessed cycle on your screen to switch between your active apps.
With Loopr open, you can go through the basic options to change the sensitivity and positioning of your triggers. You can also select a transition effect for opening apps.
With a simple sliding motion inward from the side of your screen, you can now easily switch apps while on any screen. Select the plus sign, then bring it back to center to cycle through more open apps.
Unlocking all the extra features costs $2.73 via an in-app purchase, but depending on what you're looking for, you might not even want to upgrade. You only get extra open effects and icon styles, but the one extra feature I do like—live preview—lets you hover over an icon in the cycle and see a full screen preview of it.
On the negative side, I don't that there is any way to kill apps from the cycle, but if you were looking for an alternative way of multitasking, this is worth a try.