The free SkalMac Tuner is a great, easy to use, intuative standard tuner.

This has far more non-standard tuning options, but for someone like me, who simply wants an easy way to tune their guitar I find it far more clunky and annoying to use.

But I'm not a professional musician, and I don't generally need non-standard tuning, so I won't rate this app. It might be a boon to the right person. But for many it will be overkill.