The Roadie 2 Automatic Guitar Tuner.

Buy Now - Roadie 2 Automatic Guitar Tuner

"A guitar tech in your pocket?!" 

Even though that really doesn't sound too appealing (No offence meant to all those hard working guitar techs out there).

Having a device that can accurately and quickly tune your guitar or bass, or any multitude of string instruments to any number of tuning variations  is kinda the holy grail!

Here's what the company has to say about it:
"An automated string tuner, that is quick, easy to use, & three times more accurate than the human ear. Roadie 2 adds new dimensions to music-making by allowing you to freely experiment and save alternate custom tunings."

It also comes with a companion app where you can store and transfer your alternate tunings and also download the tunings for many different instrument types.

There's also "Roadie Bass" which has a more powerful motor for the thicker heavier strings of a bass guitar and is also calibrated to deal with the much lower frequencies that a bass guitar produces. 

Having an actual guitar tech in your pocket might not be practical (or desirable), but having The Roadie 2 and the Roadie Bass close at hand will be.