Mac OS X 10.5.5 Build 9F13 Seeded
Apple appears to be sending out weekly seeds of the newest version of Mac OS X 10.5.5 to developers. The prior seed was released on August 7th.
This new version (Build 9F13) details 10 more bug fixes and unlike prior builds, Apple lists no known issues.
It's difficult to predict when the next version of Mac OS X will be released to the public, but it should be a free upgrade and include a number of bug fixes.