Apple CEO Steve Jobs will deliver his Worldwide Developers Conference keynote at 10am (6pm GMT).
Jobs is expected to reveal details of Apple's forthcoming OS X upgrade, Panther, as well as announcing a move to new processors.
A host of Mac Web sites have published screenshots reportedly of Panther. These reveal a revised Activity Monitor, the reappearance of coloured labels for files and folders, video messaging in iChat.
They also show a new System preference called Expose, which is described as a means to allow users to temporarily see all open windows simultaneously.
There is also a new browser-based Finder, an improved DVD player, and a control that lets users "attach folder actions" to folders.
The site that originally posted the images later removed them, claiming Apple's legal team had served a "cease and desist" order.
Apple will offer a delayed webcast of Jobs address after the event ends.