Requires iOS 7
CSwitcher moves your app switcher into the control center. It is customizable, allowing options of 5 icons or 4. And, it offers the ability to include app previews as well. It is simple to use, just double tap the home button and control center will show up. It also allows you to easily close apps by holding them to enter an "edit" mode then tapping their corresponding close box.
Sourece code available on Github
Configure options from Settings.