At Google I/O 2014, Google showed off a new feature in the Android 'L' build that provides heads up notifications. Basically what this does is show a toaster pop up at the top of your screen for notifications. This particularly useful when in a full screen app or game, have your notification bar hidden or in a mode where you can't easily get to your notifications.
If you don't have a Nexus 5 or 7 or don't want to install the full developer build of Android L but want this same feature now, then Heads Up! is the app to try. After installing the app, hitting Start (to enable notification access) and rebooting your device, this app will display Google Play Games style toaster popups for your notifications. Swyping left or right will clear the notification but that is all (so no archiving in Gmail or marking a text message as read in Hangouts) so when you get to that app, the item still shows up as new.
The most subtle feature is the ability to only have the heads up notifications show up in full screen mode or when the notification bar is hidden so you don't have to see the same notification twice or if you only want the toaster notification in those instances.
Currently the app works only in Android 4.3 and above due to various system requirements. The app is free to check out on xda but if you want to easily get regular updates and/or support the developer, be sure to purchase the app from Google Play.