Apple announced OS X 10.8 Mountain Lion as the successor to OS X Lion and as the next iteration of Mac OS X back in February. Around two months have gone by since the upcoming update was announced and Apple seams to be making quite some progress as they have release the third Developer Preview (build 12A178q)
This release aims at fixing several bugs and improving the performance from the previous release. Apple has still a lot to do before the OS is ready for public consumption in Summer. OS X Mountain Lion will bring several features from iOS including the Game Center and iMessages to Apple’s desktop platform.
For now, if have signed up for the OS X Developer Program you can download the OS X Mountain Lion 10.8 Developer Preview 3 right now.
source - Apple