Found this neat guitar tuner APP on the Fender website. Free for iPhone or iPod Touch. Seems to work well, and give you the options for alternate tunings, 22 of them, including open G, open F and DADGAD.
ELECTRIC, ACOUSTIC, BASS
Choose your instrument: Plugged-in guitar or bass, plus acoustic guitar.
22 TUNING CHOICES
Go with standard EADGBE, or pick your favorite alternate tunings, including open G, open F and DADGAD.
AUTO OR CHROMATIC
Use the string-by-string guide â€“ or go chromatic and tune to any note on the scale.
Download and info here, http://intl.fender.com/en-GB/features/tuneapp/