Well, you could argue that any one of the OS X ‘Big Cats’ was the winner, simply because Apple has made a user-friendly UNIX system that outperforms most, if not all, other operating systems. Leopard is now fully POSIX conforming making it more inter-operable with other POSIX systems.
Some other goodies are things like DTrace which will help hardcore C hackers more than those who are building widgets, but still, there must be some hardcore C hackers out there eager to build things for the Mac, and hopefully not just device drivers, though those are welcome.
I look forward to using XCode 3. XCode has always been one of the best IDEs out there, not really fair to call it an IDE since it is so much more fundamental to Apple development than just an IDE.
Sadly, Apple seems to have neglected perl. Though perl 5.8.8 is going to be included in Leopard, 5.10 is due out shortly. (It has been due out shortly for a long time now.) Also, Apple completely neglects mention of perl on their fancy 300 Leopard features page. There is one mention of perl in association with DTrace but otherwise Ruby and Python get all the attention. With so many perl developers carrying around Apple laptops and so much Apple or Mac specific software on CPAN you would expect more and better perl integration in Leopard, but you would disappointed like me.