Running multiple apps side-by-side is something that many users – especially those with tablets – have wanted on Android for a long time. And while we've seen a few implementations before (remember Cornerstone?), none have really taken off. Sure, Samsung has an option for multi-window on its more recent devices, but that's still a far-from-perfect solution, as it only allows certain apps to run together.
Given how oft-request/desired/lusted after this feature is, the devs behind the Paranoid Android ROM decided to try to bring it to life in a practical, usable way. However, it's probably not yet exactly what you're expecting, as it's little more than just pop-up windows with containerized apps held within. They released a tech demo showing off how it's functioning thus far, though not without noting that this is just a "first draft" and only shows about "10%" of what the finalized version will be capable of. Thus, watch the clip with an open mind.
The Google+ post highlighting the feature goes on to mention that this is far from the final implementation of the way apps will be launched, and calls the notification popouts a "cheapo plan B" solution in case "what [they] have in mind" doesn't work out.
If they can deliver on the hype being currently built around this feature, then it's definitely something to look forward to. Keep an eye on Paranoid Android's Google+ page for further developments.
Paranoid Android (Google+)