When Horizon 1.0 shipped just a few months ago, what set it apart as a calendar app is that it pulls in the weather forecast of the location of your upcoming events. It also gives you an inline forecast view for the upcoming 2 weeks.
Today, version 2.0 was released and it adds natural language parsing, an updated icon, and some other nips and tucks.
There are some extremely great calendar apps for the iPhone (Fantastical, Agenda). If you’re a calendar nerd, or just curious, Horizon is certainly worth checking out.