Previewed at WWDC, launching in the fall.
Apple Seeds Mac OS X Leopard (9A527/9A528a) [Updatedx4]
The latest seed is listed at build 9A527 (client) and 9A528a (server), and reportedly list a number of less significant "known issues" compared to prior seeds.
MacScoop had previously reported that they believe that Leopard was quickly approaching Final Candidate status targetted for late August/early September, after which there would be four to six weeks of "intensive testing" before reaching "Golden Master" status.
Meanwhile, Appleinsider notes that Apple is requesting specific testing feedback, such as Time Machine, Mail's notes, 'To Do' items, and iChat file transfer functions.
Update: ThinkSecret notes some obvious changes to the newest version of Leopard:
Sources say Apple has included a new intro movie to Leopard with the build, removing the Spotlight-like movie from 10.4 Tiger and instead providing a sci-fi space theme one. The default desktop picture is also space themed, making Time Machine's roll in Leopard's feature set apparent.