[andrewrice]

This reminds me of the days of transferring data using the 2.5mm jack on the TI-83.

[roywiggins]

Older(?) Canon(?) DSLRs also used a 2.5mm jack to control the shutter release, so some people realized if you just spammed data into it from the TI-83 you could trigger it. Add a crude BASIC script and you have an extremely basic timelapse setup.

[voltagex_]

I didn't realise how common it was to multi-purpose these jacks until recently.

Nexus 4 and 5 have their UART via the headphone jack - you need a TRRS headphone cable with the correct resistor wired up to one of the pins.

Kenwood even came up with a standard for it: http://www.miklor.com/COM/UV_Technical.php#progcable

A popular series of VHF/UHF