This is a nice piece of recent research: Keystroke Recognition Using WiFi Signals
That link goes to the ACM Digital Library, where the paper is behind a paywall. Oddly, it is also available on the ACM SIGMOBILE site without a paywall: https://www.sigmobile.org/mobicom/2015/papers/p90-aliA.pdf
If I am understanding this thing correctly, WTF...they are figuring out keystrokes by looking at the variations on a WiFi signal caused by the movement of human fingers during typing, which change how the signal propagates!?
Note that the computer you are typing on does not even have to be using WiFi. Heck, you would not even need to be using a computer. This should work if you are using an old manual non-electric typewriter.