Apple is set to bring automatic app downloads to OS X 10.8 Mountain Lion when it debuts later this summer. As noted by 9to5Mac, the latest build of OS X 10.8 Mountain Lion only has a partially working Automatic Downloads feature, but users can expect to see a finalized working version when OS X 10.8 ships.
Automatic Downloads is currently a feature of iOS which allows the same app to be downloaded instantly on all a user's devices (iPhone, iPod touch, and iPad) no matter what device the user chooses to download the app on. The feature is an easy way to keep all your devices in sync with any new apps you may have purchased.
Automatic Downloads in OS X 10.8 Mountain Lion could work in a similar manner. When you first download an app from the Mac App Store a notification will appear asking if you want to enable Automatic Downloads. Doing so will install any app purchased on the Mac App Store to any Mac that is logged in with the same Mac App Store account (provided those Macs are also running OS X 10.8 Mountain Lion).
Automatic Downloads in Mountain Lion is just another way Apple is bringing the user experience of OS X and iOS closer to parity.