Today Apple launched the Mac App Store, if your Mac runs Mac OS X Snow Leopard the store can be installed as a software update (iTunes) on your computer.
Apps are available, via the iTunes interface and can be downloaded, re downloaded and installed directly from there. Access iTunes using your iTunes password then search for Apps by price, category, utility, newest and so on. Application prices vary, some are free. A brief description of the App and reviews, where available, are also shown.
A record of the Apps downloaded is maintained by the App Store. This allows Apps to be re downloaded since you already own them, which is useful if you own several Apple products.
Apple already have Apps for iPhone and iPad accessible via iTunes.