Mac Games Arcade is a digital game repository for the Mac. It is an application that allows you to browse quite a few games for the Mac. You can also download and purchase them from the application itself. Most of the games that are listed have demos that you can download before purchasing them. There are quite a few "indy" games, but there are also well-known commercial games available, like Borderlands, Postal, Civilization, and Assassins Creed, among others.
I didn't personally try purchasing and downloading a game using this app. I did, however, download and play a demo. The title I tried is Power of Defense. It was a little over 40 megabytes in size and the download speed was very good, maxing out my connection easily. When the download was finished, I could find the game under "My Demo". The game was launched and it ran quite smoothly. You get 60 free minutes of gameplay for all demos and you can then purchase the full-game.
All in all, this app is a gateway to a bunch of games. You can sort them by category or use the search feature to find the game that you want. It works very well and there is nice content to be found.