Mac games publisher Aspyr has thrown the switch on their online game store GameAgent. Somewhat similar to Valve's Steam, GameAgent allows buying and downloading games for your Mac. They seem to be starting slow with only Call of Duty 4 ($54.99) and a trivia / SAT prep game called futureU ($39.99) available at this time. But other classic Mac games like Stubbs the Zombie appear to be coming soon at $19.99.
GameAgent also offers an Extended Download Service for an additional fee of $4.99. Basically, what this does is give you the right to re-download the game anytime within two years. So, if you have a hard drive crash and no back-up, you can download again.
One downside is that the game is locked to your computer, so you can only play them on the machine they were downloaded on. If you want to re-download the games on a new computer, you must purchase the EDS service when you buy the game. Even then, a given serial number can only be activated three times.