iOS: PopCal is a simple, fast and attractive calendar app for iOS. Its interface is also fun and easy to use.
PopCal focuses on speed and efficiency and it delivers on both counts — even on my extra-slow jailbroken iPhone 4. The app is very responsive, and the QuickPop feature makes it easier to select a date or time without the need to tap several times. Instead you just tap and hold down on what you want to change, a large screen appears, you drag your finger to what you want, and simply let go. It's a simple but novel way to make adjustments.
Other than that, the app is just a very well-made calendar app with a great interface. It's definitely a contender for the best calendar app in our iPhone app directory.
PopCal ($0.99) [iTunes App Store]