My Ubuntu machine and my Nokia N900 both have something of an ‘app store’ in them.
If we call an app store – an application that connects to platform-specific software repositories to easily install software, notify me of updates, etc. Though, in the Un*x world, these things are called package managers or application managers.
The Mac has never had one of these. Most updates are handled by the software vendor – Adobe has their updater, Apple theirs, Firefox and other applications handle updating from within the application. Since OS X is BSD-based, there have been attempts to create Un*x-esque package managers for OS X – Fink & MacPorts come to mind. I’ve tried them, and found the unsatisfying.
If we say, the majority of the software will be $0 (and other definitions of free) – then the difference between a Mac App Store and a Unix Package Manager becomes a question of whether or not it’s running within iTunes.