It used to be that the average person playing a stringed instrument, often it would be a guitar, had in terms of tuning resources, a tuning fork. This at the time would be considered as the 'best guitar tuner'. In this era, more houses had pianos so it was also common to tune to the piano. In other circles a pitch pipe was used. Many tuned by ear and you can even hear this 'ear tuning' on some popular albums that were recorded at the time. All these tuning circumstances encouraged pitch sensitivity. The instrumentalist really had to focus on what he or she is hearing and tune whatever string involved accordingly.
What was initially available at the time in terms of technology was the 'strobe tuner' created by Peterson Tuners. These tuners were often used by those in the recording and touring profession but were too expensive for the average musician. This tuner was very accurate by nature and was based on technology that was actually invented in 1931. To understand how strobe technology works it might be helpful to look at how it is applied in other industries: any thing that might use a 'flashing light'. This can be found in the airline industry, movie and theater industries, today the cell tower industry and so on. That 'flashing motion' causes circular motion to appear to stop in mid motion until the next flash. Presumably this technique can be applied instead to the sound wave of a guitar string being tuned rather than a light wave and the 'stillness' effect occurs when both are matching the same frequency.
It was the 'Korg' company that came out with the first 'hand held' tuner and this would be in 1975. This tuner was at the time a usable device for all musicians with a price that fit the need. This conception at the time spawned a whole new industry where upon many companies got into the business of instrument tuning. The WT-10 had an L.C.D analog needle display and actually looks similar to a rectangle transistor radio of the time. It could be held up by a brace in the back and looked like it has a number of features on the side. Buttons pressed would accord with the string being tuned. A similar looking tuner is actually available today by the same company with added features.
Today we have many different types of tuners. We have 'clip on' tuners (this idea first spawned by the Intellitouch company), and more console tuners by various companies. We have other companies jumping into the strobe technology arena. We have tuners on a capo, tuners that also have chord references, tuners that have great 'strobe-like' displays and other tuners that have a better response time then others. Some hang on to a note longer than average making the tuner easier to use. We have tuners (usually the clip on style) designed for difficult visual or auditory tuning environments. We have also wireless tuners (exclusively at this time by Korg) with the orchestral environment in mind and tuners that are good for the budget. Many are chromatic and others are designed for a range of instruments. Some are good to use for alternate tuning needs and others for mandolin and violin pitch range. Many have a 'pitch reference' and or a metronome. Many have calibration settings.
Clearly today we have the convenience of a wide range of technology available for a wide range of needs. Because of this we have made whatever tuning situation much easier. For example setting up before a play or setting up before a gig has become less arduous. There are higher tuning standards with today's recordings that come out. People can now tune at the same time without interfering in each others process. When considering a tuner it is important to consider what your needs are in order to make the best investment. It is my hope that in all this our 'pitch' sensitivity needed for yesterday does not get neglected for the conveniences of today.