Apple's new range of iPads has taken most of the headlines after yesterday's launch event. But arguably they weren't the biggest surprise, or the best news, of the day.

In a shock announcement Apple also said that Mac OS X 10.9 'Mavericks' - the new update to its venerable operating system for laptops and desktops - is now free.

And that's not just free to users of the previous version. Every Mac computer released since the 2009 Mac Mini will be eligible for the upgrade, via the Mac OS X App Store.

But why should you upgrade? What's different and what are the key new features?

Here's a great summary to let you know what's new in Mavericks.