The Gold Master version of Apple's OS X 10.8 Mountain Lion is now available on the OS X Dev Center. Gold Master versions are the final build that ships to customers, so what we see here will be what you're using later this month when the retail version comes out for $20.
Apple also seeded a new version of 10.7.5, which will likely be released the same day, with updates for folks who don't upgrade to Mountain Lion. We'll have full change logs and updates to what's new in the builds soon. [9to5mac]