A few weeks ago Apple made an announcement which surprised a lot of people. They have decided rather than charging for their new Mac OS 10.9 aka Mavericks they would make it free of charge to any customer whose Mac has at least OS 10.6.8 and meets the minimum system requirements. Additionally they announced that any new Mac purchased on or after October 22nd 2013 would qualify to receive Pages, Numbers and Keynote (formerly known as iWork) for free. Previously these applications were $20 each. Both Mavericks and the former iWork applications can be found in the App store right on your computer.
If you have purchased a new computer on or after October 22nd, 2013 and those Apps are not showing up as free in the App store, just enroll in the Up-To-Date Program for Pages, Numbers,and Keynote here http://www.apple.com/creativity-apps/mac/up-to-date/