Amazon announced a new pricing for SimpleDB today. Here is an extract from their pricing page:
You can get started with SimpleDB for free and without risk. For at least the first six months of the public beta (starting December 1, 2008), you pay no charges on the first 25 Machine Hours, 1 GB of Data Transfer, and 1 GB of Storage that you consume every month. In most use cases, approximately 2,000,000 GET, QUERY, or QUERYWITHATTRIBUTES API requests can be completed per month before incurring any usage charges. Many applications should be able to operate perpetually within this free tier, such as a daily web-site analysis and traffic reporting tool, a web indexing service, or an analytics utility for online marketing programs.
This is significant shift from the earlier approach that they have taken with all other Web services, where they did not offer any free trial.
SimpleDB Explorer is free for 30 days, now SimpleDB is FREE for 6 months for upto 2,000,000 requests, so its even easier to give SimpleDB a try.