Here at Android Police, we're no strangers to digging around in Google's code and finding surprising stuff inside. Apparently some members of the CyanogenMod team did the same, and found a hidden feature in KitKat: Heads Up notifications. These floating notifications are meant to be used in full-screen apps or Immersive Mode, but for whatever reason, they aren't switched on in AOSP code. (Perhaps they're intended for the next major Android release.) You can probably guess what happens next.
Starting with CyanogenMod 11 nightlies published on June 19th, the new builds include a setting to enable Heads Up. When switched on, all notifications will float in just below the notification bar, or just where it would be if you're currently in full screen mode. These are basically identical to the notification you'd see if you pulled down the bar - you can tap it to go to the relevant application, swipe left to dismiss it, pinch out to expand, et cetera. All the standard functions should work, including embedded action buttons, though you can expect some teething troubles as the nightlies progress.
CyanogenMod also includes one feature that wasn't in the original code: the ability to select which apps will show notifications and which won't when Heads Up is enabled. You can get to all of this in the new Heads Up section of the primary ROM menu. Expect it to get a wider release in the next monthly "M" release of the popular custom ROM.