If you have an iPhone or iPod touch you never need to worry about buying an expensive tuner every again. For £1.19 you can pick up eTuner from the iTunes App store.
eTuner is an interactive chromatic tuning application, which utilizes the quality audio and fine display capabilities of the iPhone or iPod touch to determine and display musical pitch information as you plan an instrument or sign a note.
eTuner works on a variety of instruments including wind, brass and string. Other instruments that produce a sustained tone will also work fine.