Another month, another Amazon web service...this time it's Amazon SimpleDB, which Amazon announced yesterday, and will begin limited beta testing in the next few weeks. This is yet another reason that startups don't need a datacenter, hardware, Oracle licenses, a DBA, etc.
A simple REST interface lets you store and retrieve attribute-value pairs in the cloud. While Amazon S3 is ideal for storing large files (e.g. photos, videos, audio files), you could use SimpleDB to store all the data your web application needs to run. A SimpleDB attribute could also be a pointer to a file in S3. What a great way to provide persistent storage for a web application without having to use SQL or worry about schemas.