If you have an iPhone (or an iPad), check out GuitarToolkit. I think it's freakin amazing. It's a tuner, and has settings for 6-string, 12-string, Uke, Banjo AND alternate tunings (drop-D, etc.) It also has a metronome, scales, and a chord finder.
It's $10. I put it on my iPhone, and then was pleased that it was also placed for free on my iPad. With the bigger screen, the iPad version is awesome.
The chord finder is invaluable... after all: quick: how do you play a C#m7? It's there.
Sometimes I get somewhere without a guitar tuner, but I always have my iPhone. GuitarToolkit saves the day.