Apple has released the gold master build of OS X 10.8 Mountain Lion to developers. This is the final build of the operating system before release to the public. In general, a gold master is exactly the same as the .0 version of a release, may between now and the actual release date, there may be some minor changes.
The term gold master comes from the recording industry, where a gold disc would be sent to record plants to press vinyl records. For operating systems, this would be the final build that would be sent to CD plants for packaging. The delay between the release of the gold master and the public release could be several weeks. Since OS X will only be released digitally, the delay no longer matters, but developers still need a final version against which to test their software.
Apple has not specified the release date for Mountain Lion, but the company has an earnings call on July 24, so there is speculation that OS X 10.8 will be released in the days following that call.
Mountain Lion will cost (only) $20, and will be available only through the Mac App Store.