Tuners can be as simple as a tuning fork, pitch pipes, or a device (like the Shar SM1000 metronome) that can generate tuning tones. The Cherub Keytone (item #C62) is an example of taking the pitch pipe concept to a more convenient electronic product. "Direct Connection" vibration-sensing tuners are very popular with acoustic instrumentalists for their ability to perform with accuracy in noisy ensemble environments.
  Performers of ancient music on period instruments will be very interested in electronic tuners that are capable of being recalibrated to the tuning standards of the period in question, such as the Shar Wide Range Digital Metro-Tuner (item #ST204). Teachers who are proficient on multiple instruments will require products that have more extended tuning ranges to accomodate all instruments in their arsenal, while students will be able to get by with something simpler.