Leopard will officially support Ruby & Python bindings in Cocoa which is terrific news. I'm a big fan of interpreted languages when appropriate.
Of course, there are about a dozen other very cool technologies for developers in Leopard that should really make application development, faster, better, and more fun. I've gotten a chance to play some with Leopard previews, and the actual User-facing improvements are minimal. But what users will eventually get will be lots of better third-party applications. (via Theocacoa and Michael McCracken)
It's great to see how much effort is going into not just creating slicker UI's, but better supporting the people that really make or break an operating environment - developers. Microsoft has had a lot of developer support for quite awhile, and I constantly hear about various .NET meetings/presentations/technologies etc. Apple really kicked it off by releasing XCode for free (no 'professional versions' need apply) and then having terrific Developer Documentation and examples.
The Leopard developer page makes some odd claims:
Mac users love to exchange quick messages, have video conferences, and collaborate on each other's desktops across the network.
um... right, ok. I just love collaborating on your desktop. Anyways, I can't wait to dive in and start developing on Leopard.