Apple provides a pretty efficient mail app for iOS, but 3rd parties are always trying to be better and some users are always looking for more. Inbox Cube is a free iPhone app (it's not universal but will work on the iPad) that makes some improvements on the basic Apple Mail app.
Inbox Cube supports iCloud mail, AOL, Yahoo and Gmail. One feature I think is very strong: The ability to just see photo, video, or document attachments in a single click. That includes attachments you have sent as well as received.
The on-screen display is attractive, and it is easy to navigate. You get notifications of new email just as fast as you would with the Apple mail client. I had them both activated and heard the mail a
