I’ve owned a lot of tuners back in the day, and I still use a Korg tuner I bought many years ago as well as a variety of modern tuners. There are many kinds of tuners, but the modern ones today are either very accurate strobe tuners (from makers such as Peterson or Sonic Research), tuners that can sense all the strings playing at once (polyphonic), or clip-on tuners that clip on to the headstock of a guitar and sense the vibration of strings via the clip rather than through a microphone. The clip-ons are very popular now, and incredibly cheap (<$20).
tc electronic is famous for its high-quality electronics and modern, European styling, and they’ve pioneered the polytune tuner in pedal form. But now they have introduced this technology in a very small form factor as a clip-on tuner, the PolyTune Clip.
This little tuner is amazing. And the technology that went into it is startling for so little money. With an accuracy that is an unheard of +/- 0.02 cent, it offers some truly Modern Mojo tuning for your guitar!
http://www.tcelectronic.com/polytune-clip/
By the way, Steve Vai’s guitar tech, Thomas Nordegg, loves this tuner so that’s why you’ll see it on almost all of Steve’s guitars — even while he is playing!