I second media player. When I think of one-handed tasks, this is near the top. But I'd put the interface first, because it would be annoying to have to use two hands to get to a one-handed app.

As mentioned in the other portrait thread, a solution for this is simply to have a different desktop for portrait mode. Just like the landscaped mode, it'll be customized by the user with the tasks that user is likely to use when on-the-go.

}8^O~