Screen sharing, in general, is done very well in Leopard. Although overall screen sharing has been available through both the Apple Remote Desktop software and VNC on Mac OS X for years now (albeit with no built-in viewer), Leopard has made it much more accessible.
Putting it right into iChat, including voice chat along with screen sharing and piping the data over any of several instant messaging protocols, is a brilliant solution for those "my computer isn't working right" e-mails from less technically adept friends and family.
This is a feature that most users will never encounter but will certainly feel. MacOS X Tiger was designed with a single processor in mind; Leopard, in contrast, acknowledges that computer architecture has changed within the last few years.
While multiple processors have been available on the Mac for several years, the introduction of the Intel architecture has resulted in multiple cores being the norm. Previously, developers seeking to take advantage of these cores had to go out of their way to make sure their applications were properly threaded.
Enter NSOperation. This API allows developers to optimize their code so that it automatically scales to take advantage of a machine's hardware, from a low-end Mac Mini to the high-end 8-core Mac Pro. This feature will greatly benefit everyone on the Mac platform, even though it'll never be center stage at Macworld. NSOperation, take a bow.
Leopard itself shows the presence of the new Core Animation libraries. Time Machine is probably the most obvious example, with its 3-D animated look and feel. Core Animation is a real breakthrough for other developers, however, in that it makes it rather easy for any application to be written with animated 3-D interface elements. We haven't seen the full extent of what this will mean for Mac users yet because Leopard is so new, but it is something that opens a lot of exciting possibilities for both users and developers.
Aside from whizzy 3-D interfaces, the new frameworks and APIs built into Leopard will bring us a whole new level of applications in the months to come. Leopard allows developers to easily write 32- and 64-bit applications compatible with both the PPC and Intel architectures, chock full of goodies. Application integration, iLife-esque functionality and access to the Core technologies built into Leopard help shape Apple's development environment into a powerful and easy way to develop new software. The future looks brighter than ever for the Macintosh platform.
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