Amazon's Simple DB provides real-time lookup and querying of structured data, which will presumably be stored on the S3 service. According to Amazon customers will save money because they won't have to hire a database administrator and don't need a hefty up-front investment.
Here are a few nuggets about the SimpleDB:
The service is designed to be easy to use. Customers can add data via API calls and pay for what they use. The SimpleDB also doesn't require you to pre-define your data.
Pricing is based on consumption. According to Amazon it's 14 cents per SimpleDB machine hour consumed. Here's the schedule posted by Amazon:
$0.10 per GB - all data transfer in
$0.18 per GB - first 10 TB / month data transfer out
$0.16 per GB - next 40 TB / month data transfer out
$0.13 per GB - data transfer out / month over 50 TB
Data transfer "in" and "out" refers to transfer into and out of Amazon SimpleDB. Data transferred between Amazon SimpleDB and other Amazon Web Services is free of charge (i.e., $0.00 per GB).
Structured data storage will cost you $1.50 per GB a month.
Amazon has more on its site, including a developer's guide and more specifications. Via Dave Winer.