Monitors your location and alerts you when inclement weather is heading your way. You'll receive notifications like, "It's about to start raining in ten minutes!" It's certainly one of those apps that makes you feel like you're living in the future.
Simple app which displays the estimated time of arrival to common destinations, like your home and work. These ETA's are presented in Notification Center, so that they're always just a swipe away. Super convenient!
Some people don't enjoy the built-in calendar of iOS. Calendars 5 from Readdle brings Google calendar support as well as the ability to add events through natural language like, "Lunch with Bob at noon."