[iMore.com]

It's easy to view and re-download the apps you've already purchased from the Mac App Store.
If you want to view the apps currently installed on your Mac, you can always open your Applications folder or Launchpad. But what about apps you purchased at one point but deleted, or apps you bought on another Mac? The Mac App Store gives you an easy and convenient way to look at all the apps you've already downloaded, as well as view apps you've deleted or ones you purchased on another Mac.

Full story from the iMore Blog...