Today there are a host of affordable electronic tuners available that make it much easier to tune your guitar. These generally come in two varieties.
Most tuners can be used in an audible mode (where they listent to you play the string) or electronically (where you plug in your guitar if you have a pickup installed). Popular tuner manufacturers include Intellitouch, Peterson, Korg, Boss, Sabine, Seiko and Fender.
In many situations, you will find that the audible mode won't work because there are others playing or tuning around you. One solution to this is to purchase a small add-on unit that plugs into the tuner on one end and a small pickup that clips onto your guitar on the other. These clip-on units cut out ambient noise and only listen to your instrument to tune better. The Intellitouch tuner combines this concept into one design.
Strobe (stroboscopic) tuners are the most accurate type of tuner. The original strobe tuners were mechanical devices that had a disc with a pattern on it that rotated at a specific speed. The sound produced by the musical instrument to be tuned was used to make a set lamps at the frequency of this audio signal. If the note being played is at exactly the same frequency as the spinning of the disc, then the disc appears to be stable. If the note is out of tune then the pattern appears to be moving. The more out of tune the played note is, the faster the pattern seems to move. The direction of motion indicates whether the input note is higher or lower than the reference frequency.
Peterson was a pioneer in mechanical strobe tuners and now offers a line of virtual strobe tuners that are as accurate as the mechanical ones. These are much more affordable than the mechanical types and are even available in different models such as a stomp boxe and clip-on type.