gStrings in Your Pocket
What may sound like a perverse concept is actually one of the many ways smartphones can change your life. If you play a musical instrument but don't happen to have perfect pitch (most of us, sadly), you can buy a tuner, pitch pipe, tuning fork or any number of other aids to keep yourself in tune. If you have a smartphone in your pocket, however, you also can simply download gStrings. Available in the Android Marketplace in either a free ad-supported version or an inexpensive ad-free version, gStrings will help you tune any number of instruments accurately.
Although it's certainly no replacement for perfect pitch, having a tuner in your pocket is very convenient if you're a musician. Quite a few tuning apps are available for Android, but I've used gStrings personally, and it works great: https://market.android.com/details?id=org.cohortor.gstrings.