Horizon is an interesting little app that comes from a very simple idea: To combine your weather report and your calendar in one place. That may not sound like a big innovation, but it's actually not a bad one. Horizon's simple and straightforward design (put together by a young developer named Kyle Rosenbluth) makes browsing through your upcoming schedule very easy, and matching up outside temperatures and weather with what you're doing is more helpful than not.
You can use Horizon as its own calendar app, or sync it up with any of your other shared calendars, including any you have available over iCloud. And you can grab weather from anywhere (Horizon will automatically pull in temperature readings based on your calendar event's location and time), and tweak just how the time and temperature are displayed. There's also a "night mode," which is useful both as a visual indicator of how late it is, and for opening the app up in the dark to check your next day's schedule.
Horizon is a really impressive app -- it takes a few good ideas, and then really polishes them to a nice shine. Rosenbluth has put a lot of thought into that initial premise of combining the weather and your calendar, and it shows. Horizon Calendar is available in the App Store right now for US$0.99.