Apple has released Mountain Lion at an very affordable price, if you wanna update to Mountain Lion then you just need to pay $19.99 and you can download and update your existing supported Macs, another thing which Apple has said that if you are using any other Mac with same Apple ID then you can update it too without paying extra penny.
But chances are that you are having two Macs on your home with different Apple ID, like I had. So now you have to face problem as you have to purchase again right? NO, here is the trick which you can use to install Mountain Lion on any other Mac using another Apple ID.
Open your System Preference in that Mac on which you wanna update,
Now navigate to Users & Group section,
Now Press + button to add an User,
Create new user as Administrator,
Now Switch users and sign-in with New user you had just created,
Now open Apple App Store and sign-in as with that Apple ID from which you had already purchased Mountain Lion on your another Mac,
Download and Install Mountain Lion,
Now at last delete the new user which you had created,
We had shared this trick if you are having 2 or more Macs with you with different Apple ID, don’t use this trick to update everyone’s Mac you know.