Amazon Game Downloads, still technically in beta (though open to the public), currently offers 600 titles priced at $10 and less. For any title in the store, you can download the full version and play it for 30 minutes before deciding whether or not it's worth a purchase.
As of now, it's definitely intended to be a service for casual games, but obviously Amazon will expand their reach as the market dictates in the future.
Oh, and for those of you who like freebies, Jewel Quest II, The Scruffs, and Built-A-Lot are all available for free download this week only. Live it up. [Amazon via Kotaku]