As we had earlier said that Apple will release latest version of Mac OS X, Mountain Lion on 25 July and see its here, This Mac OS X comes with over 200 changes in it. If you are running an compatible machine then you can head to App Store and purchase it for just $19.99 and install it on your computer.
If you had got your Mac on or after 11 July then you can update for free. Apple has said that it will release in roll out phase so it might take some time to appear on your App Store if its not there at this moment.
The following models can update to Mac OS X Mountain Lion :-
iMac (Mid 2007 or newer)
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
MacBook Pro (Mid/Late 2007 or newer)
Xserve (Early 2009)
MacBook Air (Late 2008 or newer)
Mac mini (Early 2009 or newer)
Mac Pro (Early 2008 or newer)
This Mac 10.8 comes with over 200 changes which you can find HERE.