The unified inbox is by no means a new development, but Sparrow is probably one of the better takes on it. The app was already a popular one on Macs, and its mobile entry seems to be no worse for the move.
Sparrow supports all IMAP accounts, and allows for file and image attachments without leaving the app. There is one button to mark all messages as read, and contacts all have faces attached to them on messages, which makes for a nice, engaging look.
Sparrow’s strength is in its clean presentation. All menus are hidden and accessed by swiping on a certain area of the screen or on a message. There is a full range of controls for Gmail accounts, including starring and archiving, and swipes can also be used to navigate through an email thread. Refreshing the unified inbox requires pulling down from the top of the screen.
Currently, the only drawbacks to the app are a lack of POP account support and push notifications, though a tacked on “yet” suggests those features are in development. Sparrow is available now for iPhone, iPod Touch, and iPad for $2.99 on the iTunes App Store.