Yes, definitely OnSong!

Saw it in use by a couple of musicians recently. One had it on his iPhone with a special attachment on the microphone stand for the iphone as he performed. Most unobtrusive. OnSOng had his lyrics and chords, and is set for a chosen screen scroll speed so you don't have to be tinkering with it during performance. He had an unobtrusive foot pedal for forwarding to his next song. Another person had it on a mini iPad. I liked the feature where you could at the touch of the screen change the key. And also you could bring up small chord shapes for several different instruments. Very impressive.
So I have designs on getting a mini iPad now.

Youtube has several good videos showing how to use the app and the foot pedal.