This app, which fortuitously doesn't require root, adds a makeshift "start menu" and access to recent apps list to your screen. Here you can launch Android apps as windows much like you would on that other operating system whose name escapes me.
Since Google hasn't officially released (or even promised to release) this feature, there's no guarantee it will work properly. It could also break or be removed in a future update. More importantly, it's largely useless on phones or small tablets that don't have much screen real estate.
Still, if you have a larger tablet or just want to try out the future of Android, this is the easiest way yet to do it.