The release ends months of waiting for Mac fans, after Apple pushed back the launch to finish development on its much-hyped iPhone.
Early reviews for Leopard have been positive with veteran technology writer Walt Mossberg calling it "evolutionary, not revolutionary".
Apple is hoping to build on recent strong sales of its Mac computers.
In the last three months, Apple sold 2.2 million Macs, up 400,000 on its previous best quarter.
The company is touting Leopard as a Vista-beater, pointing to new features not found in the new operating system (OS) from Microsoft that drives many PCs.
LEOPARD'S NEW SPOTS
Time Machine - automatic file back-up
Stacks - related files and folders grouped automatically
Spaces -keep separate desktops for different uses
Quick Look - Examines the contents of a file without having to open the related program
Coverflow Finder - flick through files and folders like album art in iTunes
Boot Camp - run Windows on a Mac
Apple says there are 300 new features in Leopard, but some of them are minor tweaks to the previous OS, called Tiger, rather than fully-fledged tools or enhancements.
Writing in the Wall Street Journal, Mossberg said: "I believe it builds on Apple's quality advantage over Windows.
"In my view, Leopard is better and faster than Vista, with a set of new features that make Macs even easier to use."
In the New York Times, technology columnist David Pogue wrote: "Happy surprises, and very few disappointments, lie around every corner."
At the MacLiveExpo, being held in London, there was a mixed response from attendees on whether they would be rushing out to buy Leopard on day one.
Many of the delegates said they would wait for the operating system to "bed down" before they bought it.
"I never buy any operating system when it first comes out. I normally wait until it has been out for six months or a year," said David Ramage, a Mac user from Luton.
|Vista has been quite a disappointment for many people and Leopard could be the reason many people make the switch to Macs |
Nik Rawlinson, editor of MacUser magazine
He added: "Tiger does what I need it to do right now. I've not seen anything in Leopard to make me want to buy it immediately."
For developers, a new operating system means having to work to ensure their programs run smoothly on the new platform.
Ben Rudolph, director of communications at SWSoft, makers of Parallels, said Leopard was a big step forward for Apple and "would continue to drive sales of Macs".
Parallels lets users run Windows and Linux alongside OS X on a single Apple machine.
Mr Rudolph said Parallels would run smoothly under Leopard, barring any last minute changes to the code released by Apple.
"If that happens, we'll release a free, automatic update to account for them very soon after Leopard's launch," he said.
Of the new features in Leopard, Mr Rudolph said he was looking forward to being able to take advantage of his Mac's 64-bit architecture.
The new OS takes full advantage of the latest generation of chips inside Apple machines, while running applications on older processors also.
"I'm also looking forward to new user-experience features like Stacks, which should help me organise my incredibly messy desktop, and Spaces, which lets me cycle between different desktops."
Nik Rawlinson, editor of MacUser magazine, said many users would get Leopard in its first few weeks on sale.
"When Tiger was launched it earned Apple $120m very quickly and all the expectations are that sales will be double that."
He added: "Vista has been quite a disappointment for many people and Leopard could be the reason many people make the switch to Macs."
He said he felt Leopard had enough new features to distinguish itself from Microsoft's Vista.
"A lot of things that were previously only add-ons in the Mac world, such as the Apple TV interface, are now integrated into the OS.
"That is competing directly with Media Center on Windows PCs. Apple has seen that Microsoft has moved forward in some areas and is responding."
A review and video overview of Leopard will be published on the BBC News website on Monday.