One of the major strengths of Android has been ubiquitous multitasking, something not many other mobile OSs have managed to implement yet - one of the major reasons being its impact on battery backup. Of course, the implementation has been an evolutionary process, with the one on Honeycomb and Ice Cream Sandwich (ICS) being regarded as the most efficient ones.
Not so long ago, Onskreen had come up with Cornerstone - a method to display multiple apps on the screen at the same time, cascaded into smaller rectangles. The software icons stays at the system bar at the right, and brings up the background applications using both the screen real estate and the theme, creating a seamless multitasking experience. However, Cornerstone is not an app in itself, and needs to be properly baked into the operating system itself.