One of my favorite parts about the new Mac App Store is that it is "buy one, run everywhere" just like iOS apps on the iPad or iPhone/iPod touch. It has always been the case that you can buy an app at the iOS App Store and use it on all of your iOS devices.
"Desktop" software has often been frustratingly limited to one or two computers. I'm thinking especially of the new Microsoft Office suite, but there are many, many other apps (especially ones from Adobe) which ties a license to a single (or sometimes two) machine.
If you buy, say, The Incident for Mac for $3 from the Mac App Store, you can use it on all of your Macs.
Simply launch the Mac App Store on your second (or third or fourth, etc) computer and look under "Purchases" and you will see all of the apps that you have purchased. Apps which are already installed will be shown as "Installed" and greyed out. Apps you can install will be shown in a black box labeled "Install." Click it and the app will automatically download and install to your /Applications/ folder.
If you want to go right to that app's page, just tap-err, I mean click! the app icon and it will take you to the appropriate page.
I'd love to see the iOS App Store be updated to have a list of purchases that I have made so I know which apps I have purchased but not yet installed. The Mac App Store's purchase history page is much better than the way iTunes manages your iOS purchases. Maybe Apple could call its next event "Back to the iOS!"