The Mac App Store brings the iTunes App Store model straight to the Mac – it’s available for Snow Leopard users through Software Update as part of Mac OS X v10.6.6.
Note: it’s a separate app, so don’t go looking for it in iTunes.
Apple software is of course in the store, from iPhoto, iMovie and GarageBand ($14.99 each) to Pages, Keynote and Numbers ($19.99 each). Aperture 3 is available for $79.99.
Our previous coverage: