If you're into guitar (or any stringed instrument in the Guitar Family) GuitarToolkit is really good.
It has a tuner with various modes for loads of instruments in the guitar family all easily laid out into sub-families so you can choose from Guitar 6, 7 and 12 strings, Bass 4,5 & 6 string, Banjo, Mandolin and Ukulele.
The graphics and interface are excellent and it's very user friendly, and just generally easy to use!
It has a fretboard tool which has life-like strings and frets that when touched, sound like they are being plucked, but best of all with this setting you can choose every imaginable chord and it will show it on the virtual fretboard and you can run your thumb along it to see exactly how it would sound if strummed. You can also choose which strings on which frets you want to be play, effectively creating your own chord and again, you can strum it to see how it will sound.
It's also got a Metronome which you can adjust to whatever speed you wants and you have a large variety of sounds if the standard bugs you too much.
Anyway, all in all, I think it's a great app for guitarist, experienced and beginners, I've stopped using my tuner now as I have this on the iPhone and it works really well, no flaws at all in the tuning.
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