While the official L Version release is months ahead, you don't have to wait 'till Fall to get this particular goodie, as enterprising developers have already ported the feature to devices running Android 4.0 and up. The best part, aside from previewing some new functionality, is that you don't have to root your device if you're running Android 4.3 or up. There are two methods to get heads-up notifications early, both with rooting (using Xposed) and without (app-based). Here they are:
The no-rooting method (only Android 4.3+)
XDA member Dr. Alexander Breen released Heads Up! (Google Play https://play.google.com/store/apps/details?id=com.woodblockwithoutco.headsup), a $0.99 app that's also available for free inside the XDA Forum. It does its thing flawlessly, and it's very much worth the dollar. Install and launch it on your device. Tap "Start" and enable "Heads-up notifications service". Voilla!
Although you're already set, consider dropping by the Settings menu to see what's in store. Every option is explained, so we won't plunge into details, but it's good to mention you can choose to ignore silent and ongoing notifications, as well as open related apps by double-tapping on their notifications. You can also set their durations from one to ten seconds. In Heads Up!'s main window, there's also a Whitelist menu. In there, you can have specific apps' alerts show up as notifications.
If everything went as planned, your notifications will now behave like they came from Android L Version. We advise you to make use of heads-up notifications only for apps you'd like to have more immediacy with. Otherwise, your home-screen can quickly become a spam-board!
The rooting method with Xposed (Android 4.0 and up)
This method assumes you have a rooted Android device with Xposed installed. If you haven't heard about the Xposed Framework yet, you can read more about it here. Anyway, getting heads-up notifications is as simple as installing the Xposed module of the same name from the Xposed Installer app. After rebooting, look for the Heads Up Notifications app in the app drawer. Compared to Heads-up!, its options are more limited. You can toggle ongoing notifications, set a delay time in milliseconds, and choose to ignore notifications from specific apps. That's pretty much all there's to it, and the end result will look something like this: