While looking through the App Store, I stumbled upon something useful about the Purchases tab. If you use the same Apple ID across two or more computers, you may logically want apps purchased with the same Apple IDs installed on those computers as well. You can do this.
Select the Purchases tab in the App Store. You have a complete list of every app you have purchased using the current account. If there are apps you've downloaded on one computer that you'd like on another computer, simply click Install and it will download on that computer.
Simple, yes, but it took me a while to discover this. It comes in handy often.
[crarko adds: This is one feature of the Mac App Store that I really do like. Along with the effective Family license scheme it implements, since I have five systems to maintain.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110217182514247