AARGL  Have a look at Corporate Newt Tuner! It's about the same principle as SkalMac Tuner BUT for people who tune their guitar differently than standard tuning (for myself, I tune it one tone lower, but it's also perfect for "open-tuners" !), and it can suits the need of any other instrument though you see a guitar picture... ;-)

Of course, as I read in some other review, you need an ear and it's a simple sine wave, but I suppose that it's time to educate your ear if you want to become a musician, uh? ;-D  
(Version 1.2)

GEEKMAN  Apologies, I meant to post this as a review. This corresponds to the comment below.  
(Version 1.2)

GEEKMAN  Yes, this tuner does nothing more than act as a digital pitch pipe. Ironically, I found it to be by far the easiest to tune to out of all the free apps and most of the shareware apps on the site.

Hands-down the best tuner is GarageBand (add a real instrument track, then click the tuning fork in the little time readout panel in the bottom). I didn't want to install the massive loop library for a simple tuner, so this is a usable substitute.

I feel bad about giving this a positive review considering the simplicity, but it really does get the job done so much better than the many automatic tuners out there, assuming you have the ear for it. So, if nothing else, kudos on hitting the small, obscure niche of instrument tuners that actually work.

My one suggestion would be to have the note stop playing when you click another note. Unless there's something I don't know about tuning, it's nothing but an annoyance to have to silence one note before you can hear the other. It'd also be helpful if it could play the guitar repeatedly - I ended up tuning to the violin so I wouldn't have to keep hitting the note button.  
(Version 1.2)

MIDTOAD  This simple app just plays the 6 notes of an open-stringed guitar (or several other instruments like piano, organ, banjo, violin). It doesn't tune your guitar, *you* tune your guitar by adjusting the string tension and trying to get your guitar's string to make the same sound as the app. If you have a bit of a tin ear, that will be hard. You might be better off in that case with a hardware tuner that tells if you are sharp or flat for each string.   
(Version 1.2)