GigaOM has a good overview of the various cloud database servers which include SQL servers  like  Amazon Web Services’ RDS which allows web developers to choose between various SQL systems such as Microsoft SQL Server and Oracle Database.  The article also outlines NoSQL services including Amazon DynamoDB and Cloudant. A reader posted in comments that RDF and semantic were missing in the list:
RDF Cloud databases and semantic web How Google and Microsoft taught search to “understand” the Web | Ars Technica Google’s Knowledge Graph derives from Freebase, a proprietary graph database acquired by Google in 2010 when it bought Metaweb. Microsoft’s Satori (named after a Zen Buddhist term for enlightenment) is a graph-based repository that comes out of Microsoft Research’s Trinity graph database and computing platform. It uses the Resource Description Framework and the SPARQL query language, and it was designed to handle billions of RDF “triples” (or entities).