Android: If you liked the look of Facebook's Chat Heads for messages, then you'll love Call Popout. It's an app that keeps you in whatever app you have open and turns any incoming call into a floating icon.
If the person calling is in your contacts, you'll get their photo photo. This way, you can quickly mute and dismiss the call, answer it or hang up without leaving the screen you were working on.
Call Popout is quite customisable, letting you select whether you want it to run for all apps or select specific apps that it is activated for. Additionally, you can select the position, size and colour of the floating icon.
Call Popout (Free) [Google Play Store]