iPhone: There's nothing wrong with the built-in calendar app on your iPhone, but it can be somewhat limiting in the amount of data it lets you see and navigation isn't always perfect. Agenda Calendar cleans up the interface and makes finding what you're looking for a breeze.
At first glance it looks like you'd be paying $1.99 for a stylish, minimalistic calendar app, and, in part, you are. What you find when you actually use Agenda Calendar, however, is that it makes navigating through your schedule really simple and natural. Like you'd expect, you can go through your days by swiping up and down, but swiping left will give you a yearly overview — something missing from the built-in app — and swiping right will give you details on that specific day (you can also get these details by tapping on a day). When you're in a specific day view, you can move back and forth through days by swiping up and down. It's all these little navigational elements that make it Agenda Calendar really pleasant to use.
$2 might be a little steep for some, as you get a calendar for free with your iPhone, but if you find it frustrating to get around then Agenda Calendar can remedy that problem.