As many have blogged about, Apple has posted a rather hefty list describing all 300 new features of Leopard. Some specifics that really got the coder in me excited:
(Highlighting emphasis all mine)
Use Ruby and Python as first-class languages for building Cocoa applications, thanks to Objective-C bridges as well as full Xcode and Interface Builder support.
Use Objective-C, Ruby, and Python programs to automate Mac applications. The new Scripting Bridge enables them to easily generate AppleEvents using a concise, AppleScript-like syntax.
Create Instruments with DTrace
Monitor system activity from high-level application behavior down to the operating system kernel, all thanks to the power of DTrace and the instrument builder.
Monitor virtually any aspect of your application with DTrace, integrated into the Darwin kernel. Java, Ruby, Python, and Perl have also been extended to support DTrace, providing unprecedented access for monitoring the performance characteristics of those languages.
While some of the things can be done now (There’s PyObjC to use Python with Cocoa), the tighter integration of Python and Ruby into Leopard make it a very appealing upgrade (Even though I rather hate the new semi-transparent menubar and odd shelf nature of the Dock).