Apple has just announced the latest version of OS X, 10.9 Mavericks, will be available as a free Mac App Store download today. In addition to the usual promise of increased speed and efficiency, the update has a number of improvements over 10.8 Mountain Lion, including a more robust notification system (which includes inline replies), better support for multiple monitors and full screen apps, a revamp of Finder, and the addition of iCloud Keychain for saving passwords.
A clip from Apple's keynote.
Many other apps have been updated, but although some are getting a boost in functionality, a lot of the work done brings the overall aesthetic of OS X in line with Apple's iOS 7. Gone are the Contacts app's fake book and pages, gone are Calendar's torn page marks, and gone is the Notes app's yellow legal pad. This is the first time Apple has distributed a major update like Mavericks without charging — 10.8 Mountain Lion was priced at $19.99, while 10.7 Lion was $29.99. The free update is available to everyone running OS X 10.6 Snow Leopard and above.
It's worth noting that the discount doesn't extend to OS X Server. The latest Server 3.0 requires Mavericks and will be available as a $19.99 upgrade from the Mac App Store.