It’s open. Somewhere. Just over 1,000 Mac apps to start. Typical Apple. You must upgrade your Mac to OS X Snow Leopard 10.6.6 and then download the Mac App Store app. From Apple’s web site:
The Mac App Store revolutionizes the way applications are installed on a computer — it happens in one step. Enter the same iTunes password you use to buy music and apps on your Mac, iPad, iPhone, or iPod touch. Within seconds, your new app flies to your Dock, ready to go. So you can spend more time enjoying new apps and less time installing them.
I like having the Mac App Store keep track of apps and provide updates, ala the iTunes App Store for iPhone apps. How does retrofitting currently owned apps work? I’ve been using MacUpdate’s Desktop app for that and it works great.