Android (rooted): Facebook's Chat Heads inspired the developers behind the popular Paranoid Android ROM to copy the system for a universal multitasking method called Halo. Now, it's available for download on Nexus devices.
For those unfamiliar with this feature, when you get a new notification, a bubble will appear over whatever you're working on. Tap the bubble to open up a floating version of the app that's trying to get your attention, and you can reply or perform any normal action without leaving the app you're currently in.
Right now, the early build of this ROM that contains Halo is only available for the Nexus 4, Nexus 7, Nexus 7 3G, the GSM Galaxy Nexus and the Oppo Find 5. This is also very experimental software, so if you decide to try it, keep in mind the risks that come from using unfinished software. Of course, it's just one more feature that makes ROMs like Paranoid Android awesome -- even on a Nexus phone.