Microsoft has started offering an on-demand data service in free beta described as follows...
SQL Server Data Services (SSDS) are highly scalable, on-demand data storage and query processing utility services. Built on robust SQL Server database and Windows Server technologies, these services provide high availability, security and support standards-based web interfaces for easy programming and quick provisioning.
Unlike Amazon's SimpleDB it is intended to give Microsoft's enterprise services and products a service delivery mechanism. One of the more interesting aspects I noticed about the service is that it is integrated with Microsoft's Sync Framework, which will enable SSDS to be a central hub for synchronisation of data form multiple and possibly very different devices to provide offline/online or "occasionally connected" functionality.
The Register has a good article on it at...
Microsoft reveals its database for the cloud
...and users can sign up for the free beta at...
http://www.microsoft.com/sql/dataservices/default.mspx
It's still fairly early days in terms of all these service based offerings, but it is good to see that Microsoft is serious about providing quality offerings on this front.