Exactly what I wanted!
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