Apple says its new Mac operating system Mountain Lion comes with more than 200 new features.
Mountain Lion includes the new Messages app, the Notification Center, system-wide Sharing, Facebook integration, Dictation, AirPlay Mirroring and Game Center, iCloud integration and much more.
Here are five things you need to know before you buy:
MESSAGES: Messages replaces iChat, and with it comes all the functionality of iMessages, which lets you send free text-like messages to and from most Apple devices.
DICTATION: Mountain Lion can convert your spoken words into text.
UPDATED SAFARI: Apple has updated its browser with more features. Apple says the changes will make your computer more compatible with your mobile devices when iOS 6 is released later this year.
TWITTER INTEGRATION: With Mountain Lion, you can tweet directly from the Notification Center.
NOTES: Now when you make changes to a note on your iPhone, it will update the note on your iPad and Mac using Apple's iCloud service.
Apple Inc. ( AAPL) said downloads of its Mountain Lion software exceeded three million in four days, making it the most successful operating system release in the company's history.
Mobile devices are already responsible for the bulk of Apple's sales and profits. Now, Apple is making the new Mac system even more like the iOS software that powers its iPhones and iPads. It's also casually dropping the "Mac" name from the Mac OS X operating software, though computers will still be called Macs, not "Super-sized iPads."
The new system, formally OS X 10.8 and dubbed Mountain Lion, went on sale at a $19.99 download from Apple's App Store. It builds on the previous system, Lion, which came out last July.
Mountain Lion is made for a world where your computer is just one of your computing devices, along with your iPhone and your iPad. Apple wants to make it easier to switch from one to the other, several times a day.
If you're a Windows user, see what Microsoft has in store with Windows 8, which comes out Oct. 26. That system also promises to work well with tablet computers, but will it be as seamless as Mountain Lion?
If you already have a Mac, you can upgrade directly to Mountain Lion only if it's running Lion or its 2009 predecessor, Snow Leopard. It took a colleague an hour and a half to download and install Mountain Lion. You can upgrade for free if you bought your Mac since June 11.
Otherwise, shell out the $20. That's $20 for all your Macs, not each one. It's well worth the price just for the integration with iCloud, and you get a whole lot more.
Associated Press contributed to this report.