If you've ever downloaded a program from the Mac App Store, you can go to the Purchases tab and redownload it if you need to—as long as the app's still available, that is.
That Purchases tab will show you a list of every app you've bought (organized by date).
But what if there's a program that you didn't like (or one you no longer need, like Lion in my screenshot above)? Just as with iTunes Store purchases, you can hide things you don't want to see in your list anymore. Plus, updates for programs you've hidden will no longer bug you. So to hide something from your list, all you've gotta do is right- or Control-click on it.
That'll reveal a Hide Purchase menu, and when you click that, the App Store will make sure you know what the heck you're doing.
Confirm your choice, and you're all done! Your app will disappear from your list of purchased items.
Look, Ma! No Lion!
If you ever need to unhide any of your purchases afterward, you'll click on the Mac App Store's Featured tab, and then choose "Account" from the Quick Links list on the right.
After you sign in with your Apple ID, you'll see your account info. Click on "Manage" under the "iTunes in the Cloud" section in order to bring back anything you've hidden.
And that's all there is to it! Now you can hide or unhide at will, and you won't have to see Lion, Mountain Lion, or anything else you've bought listed within your App Store purchases. For a micromanager like me, that's a pretty big deal.
For even more information, including how to hide purchased items under 10.7, check out Apple's support article on the subject.