Stanza 3.1 on an old iPhone 3G
I think that the very first application I installed on my old iPhone 3G was the Stanza e-book reader (please note that I link to the Wikipedia article and not to the application website – already non-existing – or to the iTunes App Store application – which may disappear at any moment).
It is a fantastic, fast, responsive and user-friendly e-book reader application mainly focused on the ePub format. However, it is dying.