One of the things that makes Android so great is the fact that its world is bi-dimensional. On one hand we've got the official thing, which is pretty great being all open source and such. Customization possibilities are endless and the user experience is very nice. On the other hand, we've got the underground Android, constantly enhanced by experienced developers. One of the biggest Android development teams, CyanogenMod, recently stumbled upon a hidden feature inside AOSP Android. The Heads Up notification setting enables floating notifications that will appear below the notification bar. This comes in very useful when using full-screen mode or Immersive mode.
For some reason, this feature was not enabled by Google in Android 4.4, but was rather hidden and disabled. Most certainly this was supposed to be enabled with the next big update. CyanogenMod wouldn't want us to wait that long and that's why they are giving us the opportunity to test this setting. If you install their latest nightlies you will be able to use Heads Up starting with June 19th releases.
Of course you will have to be rooted and have some knowledge about Custom ROMs, but if you are a loyal CyanogenMod user you'll probably have no problem using this feature.
There is also another way of enabling Heads Up. You will still need to be rooted and have installed Xposed Framework and modules. The developer who made this states that the feature is still in beta, so there might be some issues with it. Other than that, everything should work just fine. Here are the links to download the Xposed Heads Up Module
or visit the official Xda forum thread
to learn more.