Don't be left with an out-of-tune guitar ever again. Here's how to tune a guitar with your Android phone:
Step 1: Download and install the gStrings Free app from the Android Market.
Step 2: Launch gStrings, then press the Menu key and select Settings
Step 3: Select the "Optimize For" drop-down list
Step 4: Select "Guitar."
Step 5: Go back to the main tuning screen.
Step 6: Press the "Tune Auto" button to have the tuner automatically identify the notes. To tune a specific note, choose a note from the row at the top and hit the "Tune" button, located on the lower left-hand side.
That's it. Now you can easily tune a guitar with your Android phone. If your phone's mic is having problems picking up sound from the guitar, go back to Settings and set the microphone sensitivity.