Before the invention of the tuning fork by British lutenist John Shore in 1711, there wasn't a great deal of attention paid to the standardisation of tunings. Even as late as the 1970s, bands often simply tuned to whichever instrument sounded the best. It wasn't until the first wave of hand-held guitar tuners in the mid-70s that musicians started to care how sharp or flat they were.
Further innovations have made guitar tuners perfect for noisy environments such as stages or the middle of an ensemble. Clip-ons attach to the headstock of your guitar and sense pitch through the vibrations resonating up the neck, while stompboxes take a 1/4" cable and track notes as electrical current.