Apple has given developers access to the first beta of OS X Mountain Lion 10.8.5. Given the planned release of OS X Mavericks this fall, 10.8.5 could very well end up being the last major version of Mountain Lion before Mavericks ships to the public. A developer beta of Mavericks was made available to developers the week of WWDC.
OS X 10.8.4 was officially released on June 4th and it brought a bunch of bug fixes. 10.8.5 will likely focus on bugs fixes and compatibility issues as well.
- Source Apple