Google Tone

Google Tone is an experimental Chrome extension for sharing the URL of the current tab with other computers. It doesn't use Bluetooth, NFC or WiFi: it only sends audio tones. "Google Tone turns on your computer's microphone (while the extension is on) and uses your computer's speakers to exchange URLs with nearby computers connected to the Internet."

For some reason, Google requires to be logged in to a Google account and to be online. Your Google Profile name and picture are displayed next to the notification. I'm not sure why this feature isn't optional.

Google Research Blog has more information about this experiment. "Tone provides an easy-to-understand broadcast mechanism that behaves like the human voice — it doesn't pass through walls like radio or require pairing or addressing. The initial prototype used an efficient audio transmission scheme that sounded terrible, so we played it beyond the range of human hearing. However, because many laptop microphones and nearly all video conferencing systems are optimized for voice, it improved reliability considerably to also include a minimal DTMF-based audible codec. The combination is reliable for short distances in the majority of audio environments even at low volumes, and it even works over Hangouts."

The extension doesn't work all the time, so make sure to turn up your volume, turn on your computer's microphone and unplug your headphones. When it works, it's a cool way to share the current tab with all the computers around you, assuming that they use Chrome and this extension is installed.