It’s not an official release from Google itself, but it’s as close as you’re going to get without hacking at the moment. Here we’re seeing the Android L heads-up notifications available as an app, created by app developer Dr. Alexander Breen. This fellow has "backported" the feature to any device running Android 4.3 or higher.
With heads-up notifications from Android L, you’ll have pop-ups galore. This system is demonstrated in the video below. Pay no attention to the rooting bits, you’ll not need to use such things for now.
You’ll need to allow "Heads Up!" device access to draw over other apps. You’ll also need to go in to settings (thankfully activated automatically by the app) to enable Heads Up access to your Notifications. From there you also have access to which apps will send you such notifications and to how long they’ll last before they time out.
This app is available through Brown himself on XDA developers forums, or if you’re the slightly less trustworthy type, you’ll also find the app on Google Play. There on Google Play the app will cost you a simple dollar.
Above you’ll see several screenshots of the app. Most of the time you’ll not have to access settings, so the app will only appear as the notifications you’re getting regularly. Simple and effective.