Mac OS X 10.6 Snow Leopard, sharing it with developers alone. The good news is that things are starting to coalesce in the run-up to WWDC, with Apple just now informing developers that Snow Leopard's APIs are now frozen, with no more alterations planned before release. That means developers can work on their Snow Leopard-ready applications without much fear of Apple mucking things up with late game OS-level changes, and is a decent milestone towards what should presumably be a summer launch. The latest build also includes Chinese handwriting recognition for Macs with multitouch trackpads, similar to the functionality included in iPhone Software 2.0, and also finalizes the Grand Central architecture, which lets developers address multiple processing cores without all the know-how and complication usually required.
Update: MacRumors is also reporting that the new build includes Windows HFS+ drivers with Apple's Boot Camp utility, allowing Windows-on-Mac users to access their Mac OS X HFS+ partitions out of the box.