With this app, any device can get the heads-up notifications introduced in the new Android 5.0 “L Developer Preview”, no root required. But according to Addictivetips (and a lot of the users), this app "has done more than just replicate a feature of Android L. Even if you aren’t dying to try Android L and some of its new features, this app is worth a try".
FEATURES:
Displays the latest notification as a floating popup
Easily control which notifications to display
Quick actions: mark messages as read in one tap (Android 4.1+, doesn't work with all devices and apps)
Display the heads-up on lock screen as well
Battery friendly! (Email me if you notice significant battery consumption after installing)
Choose how long you want the notifications to display
Pocket detection / active display (causes a tiny increase in battery usage if you get a ton of notifications)
Display notifications on the lock screen until dismissed
Privacy on the lock screen
Multiple themes
Choose popup positioning and opacity
Don't display heads-up while using certain apps
Display a popup for the currently playing song
Compact/expandable mode
Full notification
LANGUAGES:
Heads-up notifications is available in 14 languages:
English
Russian (translated by Алексей Громов)
Chinese (translated by Youlun Zhai)
Chinese traditional (translated by Hsu Still)
French (translated by ozzii)
Spanish (translated by Luis Zuñiga and Andrés Cícero)
German (translated by Jürgen Wahlmann)
Italian (translated by Demetrio Carrara)
Portuguese (translated by Eric Alencar)
Polish (translated by Michał Suchański)
Serbian (translated by ozzii)
Bulgarian (translated by Misho Georgiev)
Swedish (translated by Victor Westerlund)
Norwegian (translated by me)
If you want to help translate, you can find all the strings used in Heads-up here:
http://goo.gl/Z732Ex
While it's not translated into any of these languages, it also supports notifications in right-to-left languages.
COMMON ISSUES:
If no popups appear, please make sure that Heads-up shows up in the list of running services. If it doesn't, make sure you have enabled the service, and then reboot the phone.
If your device is running Android 4.2 or earlier, and you have other accessibility services running (such as Pushbullet), they might block Heads-up from receiving your notifications. Please try to disable them if Heads-up doesn't work.
Huawei, XIAOMI and MIUI users: please allow Heads-up to display popups under Settings > Apps > Downloaded > Heads-up. It might also be necessary to reset the app data/settings.
Uninstalling: If you find that the uninstall button is disabled, go to System Settings > Security > Device administrators > Heads-up, and tap Disable.
Other: Facebook Messenger, AcDisplay and PushBullet notification mirroring conflicts with Heads-up. And no, it doesn't display more than the latest notification (after all, this is meant for instant notifications when using the device, not as a lock screen).