Notes and Reminders
Reminders and Notes are essentially direct ports from iOS, so iPad and iPhone users should feel right at home when using these task list management and note-taking apps. And the iCloud integration means Mountain Lion's Reminders and Notes can be synced with the data on your iOS mobile devices and any other Macs you may have.
Mountain Lion also renames iCal to Calendar and Address Book to Contacts, to match the existing names on iOS. In addition, Contacts gains the ability to let you control which information in your personal address card is shared with others.