So far, multitasking on mobile has been cumbersome to use. iOS is a joke with the row of icons representing recently used applications and the two-and-a-half Apple-approved background tasks. Windows Phone is pretty laughable too ('resuming...'), and while Android is technically more advanced, and exposes its multitasking better through its intents system, actual application switching is still cumbersome and restrictive. WebOS had by far the best multitasking implementation of any mobile operating system, but battery life suffered for it because it was very unoptimised.
There have been efforts to improve Android and bring better multitasking, including multi-window, to users of the platform. Samsung has a rudimentary multi-window system, for instance, but it's very restrictive and Samsung-applications-only. Paranoid Android aims to address this issue by bringing true multi-window multitasking to Android - in fact, to all Android applications. They're showing how it works in a video, but warn the video is only "10%" of what they have in mind.
"You know multi-window is hard to get right. Cornerstone failed and was buggy. Cyanogen got their asses kicked by Google for merging it. Samsung wrote a half assed implementation that works with a hand full of s-apps and breaks the rest," wrties Paranoid Android's Paul Henschel, "It's still tough and we don't assume that we can cover everything, but it's looking alright - no bad fallouts until now."
I really want proper multitasking with multi-window on Android. The ability to run two applications side-by-side is something modern tablets are ideally suited for. Many applications have both a phone and a tablet user interface, and you could easily show two phone applications side-by-side on a tablet. Paranoid Android's hack will hopefully achieve at least some of this, but of course, what we really need is an official implementation from Google.
What about Android 5.0, Google?