Android: Facebook's Chat Heads is the surprisingly clever feature with the silly name. If you prefer to use SMS but still want to use a floating messenger, HoverChat offers the same functionality in a basic texting app.
Unlike Facebook's implementation, HoverChat allows you to set which contacts will get their own floating chat bubbles when you receive a message. You can pop out a chat bubble at any time in the regular app as well. HoverChat isn't quite as intuitive at keeping a minimised bubble out of the way as Facebook — you can leave the bubble anywhere on the screen, whereas Facebook's snaps to one side of the display or the other.