Keynote is Apple's own presentation application. Like PowerPoint, it allows you to put on impressive presentations, but faster and with better end results.
Mac OS X has a great built-it search function called Spotlight that helps you find any file on your Mac. Never loose an email or a document again!
iCal is the Mac's built-in calendar applications. It's the easiest way to keep track of appointments, events, holidays and birthdays.
The Dock gives you quick access to your applications from anywhere on your Mac. It's the fastest and most convenient way of opening any application.