Via ComputerWorld:

“Leopard also offers a veritable playground for developers, with Ruby on Rails, Mongrel and Capistrano all baked into the operating system. Developers also get DTrace, which is built into the operating system’s core, allowing developers to observe, debug and tune applications in real time.”

I guess that also implies that it comes with RubyGems baked in? (Apologies for the shameless Mac-touting and link blogging.)